TECTONICS, VOL. 10, NO. 1, PAGES 77-100, FEBRUARY 1991 ...€¦ · TECTONICS, VOL. 10, NO. 1, PAGES...

TECTONICS, VOL. 10, NO. 1, PAGES 77-100, FEBRUARY 1991

INCREMENTAL 40Ar/39Ar THERMOCHRONOLOGY OF MYLONITIC ROCKS FROM THE NORTHERN SNAKE

RANGE, NEVADA

Jeffrey Lee I

Department of Geology, Stanford University, Stanford, California

John F. Sutter

U.S. Geological Survey, Reston, Virginia

Abstract. A 40Ar/39Ar thermochronologic study of muscovite, biotite, and potassium feldspar from footwall rocks of the Northern Snake Range Decollement, Nevada, was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling (uplift) history of these mylonitic rocks. Mylonitic deformation has been bracketed between 37 Ma and about 24

Ma, and minimum temperatures during ductile thinning were about 300øC. Mineral cooling ages along the east flank of the range suggest a minimum TMAX gradient of about 55øC/Ma. This steep thermal gradient is the result of a localized heat source and/or sheafing of colder rocks with older cooling histories over hotter rocks with younger cooling histories. Although mylonitic deformation may have spanned a 13 m.y. period (37 Ma to 24 Ma), deformation was not synchronous throughout the range. On the west flank of the range, age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325øC and deformation had ceased by 30 Ma, while on the east flank of the range, lower plate rocks were at temperatures above 325øC and ductile deformation was still ongoing. This is also reflected in chrontours of published K/Ar muscovite ages, along with 40Ar/39Ar ages, which monotonically decrease in age from west to east. These data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 Ma. These relations suggest a lateral thermal gradient, the east flank of the range cooling more

1Now at Department of Earth Sciences, Monash University, Clayton, Victoria, Australia,

Copyright 1991 by the American Geophysical Union.

Paper number 90TC01931. 0278-7407/91/90TC-01931510.00

slowly than the west flank. Alternatively, the data suggest either an eastward dip of lower plate units prior to the onset of Tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping, top-to-the-east shear zone. Both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms. The 40Ar/39Ar data, along with published finite strain, structural, and quartz petrofabric data, provide constraints on the formation and geometric evolution of metamorphic core complex detachment faults.

IN'I•ODUCTION

Metamorphic core complex detachment faults have been the subject of much study over the last 15 years; however, the origin and nature of these faults and the mechanisms by which they translate midcrustal rocks to the surface are still topics of much debate [cf. Miller et al., 1983; Gans et al., 1985; Wernicke, 1981, 1985; Davis, 1983, 1987; Buck, 1988; Wernicke and Axen, 1988; Lister and Davis, 1989]. Geochronologic and thermochronologic investigations have proven to be critical in understanding the timing and conditions of ductile deformation and cooling (uplift) of lower plate rocks. Following the earliest work of Compton et al. [1977], who documented a Tertiary age for mylonitic fabrics in the Grouse Creek-Raft River metamorphic core complex, U/Pb geochronology has now documented the existence of deformed Tertiary granites in most of the core complexes, thus placing a maximum age limit on mylonitic fabrics in lower plate rocks formed during extension [e.g., Reynolds, 1985; Wright et al., 1986; Wright and Snoke, 1986; Miller et al., 1988]. Fission track and 40Ar/39Ar thermochronologic studies have been carded out in several of the core complexes and have documented the cooling history and moderate to rapid uplift of mylonitic midcrustal rocks [e.g., Blackwell et al., 1984; Dallmeyer et al., 1986; DeWitt et al., 1986; Dokka et al., 1986; Davis, 1988; Foster et al., in press]. Although

78 Lee and Sutter: Thermochronology of Mylonitic Rocks

problems have been encountered utilizing the 40Ar/39Ar technique on mylonitic rocks that were deformed at greenschist facies conditions (i.e., complex argon loss patterns and presence of excess argon have been documented), thermochronologic studies represent the most important method of constraining the age and temperature of mylonitic deformation and the cooling history of ductilely extended lower plate rocks. Thermochronologic data, in conjunction with the structural history and geometry of footwall deformation, can provide powerful constraints and insight on the conditions of formation and geometric evolution of metamorphic core complex detachment faults.

This paper presents new and previously published 40Ar/39Ar thermochronologic data coveting a broad portion of lower plate rocks in the northern Snake Range metamorphic core complex, east central Nevada (Figure 1). The northern Snake Range is ideally suited for such a study because: (1) There is minimum erosion of upper plate rocks so that the location and distance of lower plate rocks with respect to the detachment is known everywhere across the range, (2) The magnitude of finite strain and kinematic history of deformation related to the development of the detachment fault are well understood [Lee et al., 1987], and (3) Canyons that deeply incise lower plate rocks provide excellent structural relief and thus three-dimensional control for

the analysis of data from the lower plate. The 40Ar/39Ar thermochronologic data presented in this paper shed light on the timing of a Late Cretaceous metamorphic and deformational event and constrain the timing and conditions of deformation related to Tertiary ductile extension. In addition, the thermochronologic data provide evidence for steep thermal gradients during Tertiary mylonitic deformation and differential cooling of lower plate rocks.

GEOLOGY OF THE NORTHERN SNAKE RANGE

East central Nevada was the site of accumulation of greater than 14 km of continental shelf sediments consisting of an upper Precambrian to Lower Cambrian sandstone and shale sequence and a Middle Cambrian to Permian predominantly carbonate sequence (Figure 2) [Stewart and Poole, 1974; Hose and Blake, 1976]. First in the Jurassic, and then in the Cretaceous, upper Precambrian and Lower Paleozoic rocks at deeper structural levels were metamorphosed, penetratively deformed, and intruded [Miller et al., 1988], but Upper Paleozoic sediments at supracrustal levels were gently folded and cut by minor displacement thrust faults [Armstrong, 1968, 1972; Gans and Miller, 1983; Gans et al., 1987]. In the Tertiary, east central Nevada underwent 250% extension in a WNW-ESE direction [Gans and Miller, 1983]. Supracrustal rocks were extended along one or more generations of high- angle normal faults that penetrated to depths of 6 to 14 km. The oldest dated normal faults occur in the Egan and Schell Creek ranges where syntectonic volcanic rocks indicate movement at about 36 Ma [Gans et al., 1989]. There are no age constraints on the cessation of faulting, although faulting did continue until at least 24 Ma, as the youngest set of normal faults in the northernmost Snake Range cut and moderately rotate a tuff of this age [Gans et al., 1989]. Locally, Quaternary alluvial fans are offset along range-front faults, indicating that extension continues to the present day.

Studies of midcrustal rocks in east central Nevada suggest that extreme lateral and vertical thermal gradients existed in both the Mesozoic and Cenozoic [Miller et al., 1988]. In the

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Fig. 1. Index map of east central Nevada showing the locations of the Deep Creek, Confusion, northern Snake, Schell Creek, Egan, and southern Snake ranges, and Kern Mountains.

Egan Range, Schell Creek Range, and Deep Creek Range, for instance, low Tertiary thermal gradients led to the preservation of Mesozoic metamorphic and deformational fabrics [Rodgers, 1987; Miller et al., 1988], whereas in the northern Snake Range and locally in the Kern Mountains and southern Snake Range, higher temperatures were associated with ductile extensional fabrics and intrusion of Tertiary granites [Miller et a1.,'1988]. The high strains associated with Tertiary ductile deformation dramatically thinned and stretched metasedimentary units and widely overprinted older Mesozoic fabrics.

Lower plate rocks of the Northern Snake Range Decollement

Lee and Sutter: Thermochronology of Mylonitic Rocks 79

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Fig. 2. Simplified stratigraphic column of miogeoclinal rocks exposed in east central Nevada. Sources of data are Misch and Hazzard [1962], Gans and Miller [1983], and Rodgers [1987].

(NSRD) record all three metamorphic, deformational, and intrusive events described above. Evidence for the first

metamorphic event is best preserved along the southern flank of the northern Snake Range where upper Precambrian and Lower Cambrian pelites are intruded and contact metamorphosed by a mid-Jurassic plutonic complex (Figure 3). The growth of biotite + muscovite + garnet + staurolite + andalusite + corderite + kyanite in the aureole of this pluton constrains metamorphic pressures for this event to be close to the aluminum silicate triple point (3.7 kbar) [Holdaway, 1971], which is consistent with estimated stratigraphic depths for these units [Gans and Miller, 1983].

The second metamorphic event, of Late Cretaceous age, affected a broad portion of the lower plate. A series of mineral-

in isograds in upper Precambrian pelitic units trend east-west and demonstrate an increase in metamorphic grade both northward and with depth in the range. Precambrian units with greenschist facies assemblages (muscovite + biotite + chlorite) in the Hendry's Creek area can be followed continuously northward and increase in grade to upper amphibolite facies (staurolite + garnet + kyanite) in the Smith Creek area (Figure 3) [Geving, 1987; Huggins, 1990]. Farther north, Precambrian units are no longer exposed, the age of the youngest rock units beneath the NSRD decreases, and units as young as Upper Cambrian in age (i.e., the Notch Peak Limestone) were metamorphosed to upper greenschist or lower amphibolite facies in the Late Cretaceous (Figure 3). Peak metamorphic mineral assemblages include calcite + quartz + clinozoisite +


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Fig. 3. Simplifted geologic map of the northern Snake Range. Sources of mapping are Miller et al. [1983], Gans et al. [1985], Lee [1990], and Huggins [1990]. A-A' indicates location of cross section in Figure 4, and B-B' indicates the location of cross section in Figure 5.

biotite _+ plagioclase _+ tremolite ___ sphene in the Middle Cambrian calc-schist units and biotite + clinozoisite +

muscovite + quartz + plagioclase _+ calcite _+ sphene in the Upper Cambrian calc-schist units. A strong Tertiary deformational and metamorphic event largely obliterates

evidence for older deformational fabrics except in the northwestern part of the range where Tertiary fabrics decrease and die out and Cretaceous fabrics are present. On the northwest flank of the range, the Cretaceous deformational fabrics in the Lower Cambrian pelites consist of a well-


developed west dipping foliation (S 1) and associated NNW-SSE trending bedding-foliation LOX1 intersection lineation that are syntectonic with respect to growth of garnet, biotite, and muscovite. The strain associated with this fabric in the

quartzites is low where slightly flattened quartz grains and aligned biotite and white mica flakes define the foliation; the strain is higher in the schists where the foliation is defined by aligned coarse-grained muscovite and biotite. In overlying, less competent Middle .and Upper Cambrian marbles and calc- schists, this fabric is a compositional layering produced by plastic flow of calcite and transposition of bedding during metamorphism.

U/Pb dating of metamorphic monazites from upper Precambrian schist units in the Smith Creek area, as well as detrital zircons and metamorphic monazites from Precambrian schists in Hampton Creek (Figure 3), yields a metamorphic age of 78 Ma • and Fischer, 1985; Huggins and Wright, 1989; Huggins, 1990]. Abundant muscovite-bearing pegmatites in the Smith Creek area, inferred to be coeval with the peak metamorphic event, are dated as 82 Ma (U/Pb on monazite) [Huggins and Wright, 1989; Huggins, 1990]. Hence, based on the available data, the Late Cretaceous metamorphism in the northern Snake Range is at about 78-82 Ma.

Lower to upper greenschist facies metamorphism of Tertiary age affected much of the lower plate, causing retrogression of older Late Cretaceous metamorphic assemblages. This metamorphic event was accompanied by ductile thinning and stretching of lower plate units, resulting in a subhorizontal, bedding parallel foliation and WNW-ESE trending mineral elongation lineation. In the southern part of the range, mesoscopic structures and finite strain measurements indicate a dramatic west-to-east increase in strain from a low on the west

of 6:1 (X:Z) to a high on the east of 600:1 (X:Z) [Miller et al., 1983; Lee et al., 1987]. Quartz microstructures and c-axis fabrics, along with finite strain measurements, document a complex extensional strain history of early coaxial strain, exposed in low-strain rocks on the west flank of the range, overprinted by deformation with an eastward increasing component of top-to-the-east noncoaxial strain. In the northern part of the range, which was not included in the above analysis, strain associated with this fabric also increases from west to

east. The mylonitic foliation, however, is not everywhere parallel to bedding and, as described above, actually dies out in the northwestern part of the range. In the Eightmile Canyon area, a low-strain, west dipping foliation (S2), NNW-SSE trending LOX2 intersection lineation and weakly developed WNW-ESE trending mineral elongation lineation overprint Late Cretaceous fabrics and weakly deform a pegmatitic to aplitic intrusive body (Figure 3). Strain associated with the S2 foliation and mineral elongation lineation increases southward along the west flank of the range such that by the west central part of the range the foliation is at low angles to subparallel to bedding and the LOX2 intersection lineation has been rotated into parallelism with the WNW-ESE trending elongation lineation. Based on these geometric relations, the S2 foliation and LOX2 intersection lineation are inferred to be Tertiary in age [Lee, 1990]. In the north central part of the range, Late Cretaceous compositional layering in Middle and Upper Cambrian carbonates has been folded into a map-scale, recumbent isocline that is associated with a subhorizontal, axial planar foliation. Strain associated with this subhorizontal axial planar foliation increases eastward such that (1) this foliation becomes mylonitic and a WNW-ESE trending mineral elongation lineation develops in the foliation, (2) both upright

and overturned limbs of the fold are dramatically thinned, and (3) the Late Cretaceous metamorphic compositional layering has been transposed into parallelism with the subhorizontal mylonitic foliation [Lee, 1990]. A swarm of muscovite- bearing rhyolite porphyry dikes, which yield an 40Ar/39Ar age of 37 Ma (see below), intrude Middle and Upper Cambrian metasedimentary rocks in the north central part of the range, yet they are not obviously folded nor do they contain the axial planar foliation (Figure 3) [Lee, 1990]. To the east, where strain is higher, compositionally similar dikes are clearly deformed by the axial planar mylonitic foliation. Based on these relations, the isoclinal fold is inferred to be Tertiary in age [Lee, 1990].

Metamorphic grade associated with the mylonitic fabric increases both with depth and from south to north along the east flank of the range. At high structural levels there is new growth of chlorite and white mica in the pressure shadows of older porphyroblasts, and at deeper structural levels there is new growth of muscovite and biotite in these pressure shadows. A change in the deformation textures in quartz with depth indicates an increase in metamorphic grade as well. At high structural levels, quartz grains are characterized by elongate, ribbonlike grains with few annealed grains, whereas at deeper structural levels, quartz grains are characterized by recrystallized, subequant to slightly elongate annealed grains. A slight increase in metamorphic grade towards the north is suggested by new growth of biotite instead of chlorite in equivalent horizons and the involvement of higher stratigraphic units in this deformation towards the north (Figure 3). Muscovite- biotite geothermometry and muscovite-biotite-chlorite geobarometry for this metamorphic event from the upper Precambrian and Lower Cambrian pelites in the Smith Creek area demonstrate temperatures of 463 ø + 46øC and pressures of 2.9 to 3.1 kbar [Huggins, 1990]. These pressure estimates are consistent with the arguments presented by Gans and Miller [1983] that, at the onset of Tertiary extensional deformation, structural depths were equivalent to stratigraphic depths. Data presented here suggest that mylonitic deformation began at <37 Ma and that mylonitic rocks cooled below about 325øC at 24 Ma, ductile deformation ceasing shortly thereafter.

THERMOCHRONOLOGIC RESULTS

Introduction

The 40Ar/39Ar age spectrum technique has been well documented for biotite, muscovite, and potassium feldspar [e.g., Dallmeyer and Sutter, 1976; Harrison and McDougall, 1980, 1982]. Because each mineral has a characteristic blocking or retention temperature to argon, the 40Ar/39Ar thermochronology technique is well suited to decipering the thermal history of a region that has suffered multiple heating events [e.g., Sutter et al., 1985]. In this study we have analyzed 18 muscovites, six biotites, and two potassium feldspars to elucidate the Cretaceous and Tertiary thermal histories of lower plate rocks of the NSRD. In our discussion of age spectra we use the term weight-average plateau (WAP) age if 50% or more of the total released argon from two or more adjacent heating increments exhibit the same age [Fleck et al., 1977]. Plateau ages are interpreted as either the time of crystallization or time of cooling through the blocking temperature of the potassium-argon system some time after


crystallization or recrystallization. An age spectrum that does not exhibit a plateau is discordant or disturbed and may or may not be interpretable depending on its complexity. Sample localities and age spectra of dated minerals are shown in Figures 4, 5, 6, and 7, and Plate 1. Isotopic analyses are listed in Table 1. Detailed petrography and structure of the rocks discussed are given by Lee [1990]. Analytical techniques for this study are those of Lee et al. [ 1987].

Late Cretaceous Metamorphism and Deformation

Samples JL2-123, JL3-62, and JL-MW3 are magnesium-rich biotites (Ann45) [Lee, 1990] collected from calc-schist interbeds within the Upper Cambrian Dunderberg Shale along a west-to-east transect across the northern part of the range (Figure 4; Plate 1). Coarse-grained biotite porphyroblasts and clinozoisite + muscovite + quartz + plagioclase + calcite + sphene grew during peak metamorphism. Compositional layering produced by deformation synchronous with this metamorphism is overprinted by the younger mylonitic foliation. In the northwest part of the range, recrystallization of quartz and calcite is related to this younger event, whereas in the northeast part of the range, growth of white mica and chlorite is related to this younger event as well. Sample JL2- 123 is from the overturned limb of the isoclinal, recumbent fold in the northwest part of the range, and samples JL3-62, from the central part of this transect, and JL-MW3, from the northeastern flank of the range, are from the upright limb of the fold (Figure 4). All three samples exhibit discordant spectra, and ages increase with higher-temperature steps, indicating argon loss. From west to east, maximum high temperature heating increment ages are 94.2 Ma, 60.5 Ma, and 72.9 Ma and minimum low temperature heating increment ages are 70.1 Ma, 41.8 Ma, and 55.6 Ma (Figure 4; Plate 1).

If interpreted at face value, these data suggest that Upper Cambrian rocks cooled below the argon retention temperature for magnesium-rich biotite (about 300øC) [Harrison et al., 1985] at >94 Ma, thus placing a minimum age limit on peak metamorphism and accompanying deformation in this region, and may have undergone argon loss at some time <42 Ma. However, the 94 Ma minimum age for peak metamorphism described above is somewhat problematic as geochronologic data from elsewhere in the range have dated peak metamorphism as 78-82 Ma [Lee and Fischer, 1985; Lee et al., 1987; Huggins and Wright, 1989; Huggins, 1990]. It seems unreasonable to propose a 10-20 Ma span for peak metamorphism across such a small distance (a present distance of about 15 km, and considerably smaller if Tertiary strain is removed), and metamorphic biotites have been known to contain extraneous argon [e.g., Pankhurst et al., 1973; Berger, 1975; Dallmeyer, 1975a, b; Roddick et al., 1980; Dallmeyer and Rivers, 1983]. Thus, until more data are available, it seems more reasonable to suspect that extraneous argon is a problem with these samples.

Tertiary Metamorphism and Deformation

Muscovite and maximum microcline, from sample 16-117, are from a pegmatitic to aplitic intrusive body located on the west flank of the range in Eightmile Canyon (Figures 3 and 4; Plate 1). These pegmatites and aplites intrude Lower Cambrian pelites that contain a west dipping foliation (S2) and WNW-

ESE trending mineral elongation lineation, which, based on geometric reasons discussed above, are thought to be Tertiary. The core of the pegmatite and aplite intrusive body is undeformed, but, locally, small apophyses emanating from this body are boudinaged in the west dipping foliation and fine- grained muscovite-bearing phases contain this foliation. Based on these relations, the intrusive body is considered to predate the foliation. Muscovite from an undeformed portion of the intrusive body yields a somewhat discordant age spectrum, which we interpret as representing minor argon loss (the low- temperature steps) and a WAP age of 48.3 + 0.3 Ma. We interpret the muscovite spectrum as indicating cooling below about 325øC (estimate of the muscovite argon blocking temperature at rapid cooling rates) [Snee et al., 1988] at 48 Ma, thus placing a maximum age limit on the formation of the west dipping S2 foliation and WNW-ESE trending mineral elongation lineation. This age is somewhat younger than total gas (TG) ages for partially reset Cretaceous muscovites from surrounding country rocks (see below) and thus may be a cooling age following crystallization. The accompanying minimum low-temperature age of 39 Ma may be the result of a short-lived, younger thermal event at temperatures <325øC that partially degassed this muscovite and thus represents a maximum age estimate for the younger thermal event. Microcline from the same sample yields a saddle-shaped spectrum that we interpret as representing a small amount of extraneous argon (low-temperature steps) and a reheating or younger thermal event (the monotonic increase in ages with increase in temperature from about 29 Ma to 44 Ma). Recent work [Lovera et al., 1989] suggests that potassium feldspars may be composed of a distribution of diffusion domain sizes, resulting in higher closure temperatures for a large domain than for a single dominant domain size. In light of this we interpret the microcline spectrum as indicating cooling below an unknown, but below 325øC, closure temperature at 44 Ma and the low-temperature age of 29 Ma as a maximum age estimate for the time of partial degassing of this sample at temperatures below 325øC during a younger thermal event. Partial degassing due to slow cooling seems unlikely as there is strong evidence for a younger thermal event elsewhere in the range (see below), although it is difficult in microcline age spectra to distinguish between slow cooling and later reheating.

Muscovite sample JL3-90 is from an extensive swarm of aphanitic, muscovite + quartz + feldspar phenocryst-bearing rhyolite porphyry dikes exposed in the northwestern part of the range (Figure 4; Plate 1). Country rocks to the dike swarm include Middle and Upper Cambrian units that exhibit two fabrics: a Late Cretaceous metamorphic or compositional layering and a younger, penetrative, subhorizontal foliation that is axial planar to an isoclinal, recumbent fold. In its westernmost exposures, the dike swarm does not appear to be folded nor does it appear to contain this younger fabric, which may be the result of the strong competence contrast between the more resistant dikes and the less resistant marbles and calc-

schists, together with the lower overall strain. However, to the east, where strain is higher, compositionally similar dikes are clearly involved in this younger deformation and contain mylonitic fabrics. In Eightmile Canyon, compositionally similar dikes also crosscut the pegmatitic to aplitic intrusive body, which yields an 40Ar/39Ar age of 48 Ma. Muscovite from the dike swarm yields a WAP age of 36.9 + 0.3 Ma (Figure 4; Plate 1), which is interpreted as an intrusive age based on the fact that country rocks around these dikes yield older, partially reset Cretaceous 40Ar/39Ar ages on muscovite. The 36.9 Ma age places a younger age bracket on the


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39AY'K Released (•)

3S

3S

o

'-' 30 -

,_ 8S

ø o •.o

1s o

1s o- ,

JLl-a19

Muscovite

TG = aGoa Ha

39Ar'K Released

10--

JL1-814A

Muscovite _

TO = 8So6 Me

_

Cpm

p•mO

p•ml

i

p•m2

3S

o

,_ 85

L.

15

p•m3

p•m4

JLl-P. 11

Muscovite TG = ;7.3,5 Ha

i • i i i [ • i i i • • i i i • i

100 0 100

SOAr K Released (•) SOAr K Released (•)

PRESENT

STRUCTURAL

THICKNESS

I II

II II

II

/ I

Fig. 7. Vertical profile of 40Ar/39Ar age spectra from Hendry's Creek. Left and right columns are as in Figure 6. See text for discussion. TG is total gas. NSRD is Northern Snake Range Decollement.

1NSRD

compositional layering and a maximum age limit on mylonitic deformation during extension.

Metamorphic muscovite samples were collected from the Prospect Mountain Quartzite-Pioche Shale contact along two west-to-east transects across the range (approximately parallel to the WNW-ESE trending extension dh'ection) to contrast the cooling (uplift) history of the west versus east side of the range and to document the timing and duration of mylonitic deformation. In the northern half of the range, muscovite samples JL-Cm, JL-Cpi, and JL2-31 were collected from the

west and east flanks of the range (Figure 4; Plate 1). Muscovite samples JL-Cm and JL-Cpi were collected on the northwest flank of the range, and each sample contains a weakly developed west dipping foliation (S2) and a NNW-SSE LOX2 intersection lineation that postdates the Late Cretaceous metamorphism. In this part of the range this foliation is locally associated with a weakly developed WNW-ESE trending mineral elongation lineation. Based on geometric reasons discussed above, these fabrics are believed to be Tertiary in age [Lee, 1990]. JL-Cm is a frae-grained schist from the basal unit

Temp., 40Ar/39Ar o C

TABLE 1. Isotopic Analyses of Dated Samples

37Ar/39Ar 36Ar/39Ar 39Ar, 40Ar, % of total %

39Ar,

16-117, Microcline (J = 0.004365; Sample 400 11.654 1.875E-2' 500 4.848 8.963E-3 600 4.101 6.599E-3 700 4.075 6.417E-3 800 4.134 5.133E-3 900 4.306 5.379E-3 1000 4.510 3.323E-3 1050 4.754 4.882E-3 1100 5.043 4.261E-3 1150 5.332 4.884E-3 1200 5.633 4.092E-3 1250 5.888 4.044E-3 1300 6.123 2.737E-3 1350 6.336 1.177E-3 1400 6.441 1.649E-3 1450 6.663 2.072E-3 1500 6.941 2.708E-3 1550 7.479 3.404E-3 1650 8.902 5.320E-3

FUSE 13.487 1.565E-2

MM-30, Orthoclase (J = 0.004178; Sample 400 36.401 2.896E-2 500 6.018 3.479E-2 600 2.877 2.521E-2 700 2.514 2.319E-2 800 2.458 2.053E-2 900 2.464 1.532E-2 1000 2.508 1.124E-2

1050 2.615 9.305E-3 1100 2.825 1.038E-2 1150 3.113 1.486E-2 1200 3.454 2.068E-2 1250 3.728 2.422E-2 1300 3.956 2.385E-2 1350 4.281 1.382E-2 1400 4.236 1.021E-2 1450 4.269 7.019E-3 1500 4.303 5.224E-3 1600 4.475 2.902E-3

FUSE 5.440 4.236E-3

16-117, Muscovite (J = 0.0041 550 18.066 650 10.484 750 8.686 850 8.754 900 8.934 950 8.912 1000 8.871 1050 8.845 1100 8.907 1150 8.771 1200 8.351

FUSE 9.251

Weight = 0.1120 g; 36Ar/40Ar Atm = 295.5) 1.804E-2 3.4 54.3 2.955E-3 6.3 82.0 1.088E-3 7.0 92.2 1.014E-3 5.9 92.6 8.403E-4 4.7 94.0 0.000E-0 4.8 96.1 6.196E-4 6.3 95.9 8.160E-4 5.5 94.9 1.130E-3 5.3 93.4 1.334E-3 5.5 92.6 1.516E-3 6.0 92.0 2.077E-3 5.8 89.6 1.896E-3 5.4 90.9 2.250E-3 6.6 89.5 2.570E-3 6.1 88.2 3.717E-3 5.1 83.5 4.623E-3 4.3 80.3 6.142E-3 3.2 75.7 1.083E-2 2.2 64.0 2.455E-2 0.7 46.2

6.620E-12 1.234E-12

1.373E-12 1.166E-12 9.131E-13 9.472E-13 1.237E-12 1.075E-12 1.034E-12 1.080E-12

1.173E-12 1.140E-12 1.063E-12 1.297E-12 1.196E-12 1.001E-12 8.360E-13 6.298E-13 4.316E-13 1.342E-13

Total gas age No plateau

Weight = 0.1052 g; 36Ar/40Ar Atm = 295_5) 7.150E-2 1.0 41.9 9.535E-3 1.9 53.2 0.000E-0 3.3 91.5 9.597E-4 4.7 88.7 8.281E-4 5.7 90.0 0.000E-0 8.3 98.2 6.022E-4 10.2 92.9 7.080E-4 9.5 92.0 8.327E-4 6.5 91.3 1.513E-3 5.0 85.6 2.052E-3 4.4 82.4 1.632E-3 3.9 87.1 1.403E-3 3.8 89.5 2.344E-3 6.0 83.8 2.339E-3 5.7 83.7 2.729E-3 5.7 81.1 1.959E-3 6.0 86.5 2.424E-3 6.0 84.0 7.045E-3 2.5 61.7

78; Sample Weight = 0.09709 g;

1.43E-13 2.90E-13 5.00E-13 6.97E-13 8.50E-13 1.23E-12 1.52E-12 1.41E-12

9.68E-13 7.53E-13 6.54E-13 5.79E-13 5.62E-13 8.93E-13 8.54E-1 8.48E-1

8.88E-1 8.97E-1 3.74E-1

Total gas No plateau

36Ar/40Ar Atm = 297.5) 0.000 2.661E-2 1.8 39.8 0.000 1.531E-2 2.2 56.8 0.000 7.718E-3 4.1 73.7 0.000 7.602E-3 7.5 74.3 0.000 8.316E-3 12.2 72.4 0.000 8.283E-3 14.4 72.5 0.000 7.973E-3 13.4 73.4 0.000 7.926E-3 13.6 73.5 0.000 8.054E-3 13.3 73.2 0.000 7.698E-3 8.2 74.0 0.000 6.475E-3 6.8 77.0 0.000 1.040E-2 2.5 66.7

3 3 3 3 3

age

2.17E-14 2.69E-14 4.89E-14 9.01E-14 1.47E-13 1.73E-13 1.62E-13 1.63E-13

1.60E-13 9.93E-14 8.16E-14 3.06E-14

Total gas age Plateau age

Apparent K/Ca,

mol/mol

27.7 58.0 78.8 81.0 101.3 96.7 156.5 106.5 122.1

106.5 127.1 128.6 190.0 442.1 315.3 251.1 192.0 152.7 97.8 33.2

18.0

14.9 20.6 22.4

25.3 33.9 46.3 55.9 50.1 35.0 25.1 21.5 21.8

37.7 50.9 74.1 99.6 179.2 122.8

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00

0.00 0.00

Apparent Age, Ma

49.1+0.9 31.0+0.4 29.5+0.4 29.5+0.7 30.3+0.4 32.3+0.4 33.8+0.4 35.2+0.6 36.7+0.9 38.5+0.6 40.4+0.6 41.1+0.3 43.3+1.1 44.1+0.9 44.2+0.7 43.3+0.4 43.4+1.0 44.1+1.3 44.3+2.0 48.5+4.0 38.0

111.6+3.9 24.0+1.2

19.7+1.3 16.7+0.6 16.6+1.3 18.1+0.5 17.5+0.3

18.1+0.5 19.3+0.5 20.0+0.5 21.3+0.8

24.3+0.6 26.5+0.9 26.9+0.4 26.5+1.0 25.9+1.0 27.9+0.8 28.1+0.8

25.1+1.5 22.7

38.7+0.4 44.3_+0.3 47.6_+0.2 48.4+0.3 48.1 +0.3 48.0+0.3 48.4+0.3 48.3+0.3

48.5+0.3 48.3+_0.3 47.8+0.2 45.9-+0.3 47.9 48.3+0.3


TABLE 1. (continued)

Temp., 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39Ar, 40Ar, 39Ar, øC % of total % mol

JL3-90, Muscovite (J = 0.004145; Sample Weight = 0.1062 g; 36Ar/40Ar Atm = 297.5) 550 6.642 2.133E-2 6.388E-3 5.1 71.5 5.34E-14 650 5.851 8.672E-3 3.792E-3 4.9 80.8 5.14E-14 750 5.510 0.000 2.182E-3 7.2 88.2 7.57E-14 850 5.376 0.000 1.092E-3 12.0 93.9 1.27E-13 900 5.352 0.(}00 1.159E-3 17.6 93.5 1.85E-13 950 5.339 0.000 1.341E-3 19.0 92.5 2.00E-13 1000 5.363 0.000 1.224E-3 13.9 93.1 1.46E-13 1050 5.453 4.366E-3 1.670E-3 9.7 90.8 1.02E-13 1100 5.662 7.794E-3 2.454E-3 6.4 87.1 6.79E-14 1150 5.897 2.333E-2 3.546E-3 3.2 82.2 3.43E-14

FUSE 7.266 1.349E-1 1.012E-2 1.1 58.9 1.12E-14 Total gas age Plateau age

MM-30, Muscovite (J = 0.003650; Sample Weight = 0.1198 g; 36Ar/40Ar Atm = 295.5) 550 16.618 2.067E-2 4.292E-2 1.5 23.7 1.765E-13 650 8.417 1.082E-2 1.500E-2 1.9 47.3 2.261E-13 750 6.358 4.984E-3 8.905E-3 3.0 58.6 3.517E-13 850 6.375 9.440E-3 6.260E-3 4.0 57.1 4.646E-13 950 6.583 3.068E-3 9.739E-3 12.1 56.3 1.415E-12 1000 6.016 2.234E-3 7.850E-3 11.3 61.4 1.321E-12 1050 5.533 2.942E-3 6.129E-3 12.0 67.3 1.403E-12 1100 5.384 0.000 5.699E-3 11.0 68.7 1.288E- 12 1150 5.489 1.079E-3 6.086E-3 12.6 67.2 1.472E-12 1250 4.935 1.338E-3 3.723E-3 21.7 77.7 2.539E-12

FUSE 5.110 4.009E-3 4.289E-3 9.0 75.2 1.059E-12 Total gas age Plateau age

JL2-123, Mg-Biotite (J = 0.004157; Sample Weight = 0.0902 g; 36Ar/40Ar Atto = 297.5) 550 13.816 1.123E-2 1.449E-2 11.9 69.0 650 13.833 3.028E-3 6.878E-3 17.5 85.3 750 13.832 0.0000 5.755E-3 10.2 87.7 850 14.029 7.614E-3 6.716E-3 5.0 85.8 900 14.262 1.181E-2 9.534E-3 4.1 80.2 950 14.028 1.447E-2 7.821E-3 6.0 83.5 1000 13.730 0.0000 5.931E-3 10.0 87.2 1050 13.729 0.0000 5.439E-3 11.1 88.3 1100 13.779 0.0000 4.618E-3 10.6 90.1 1150 13.808 0.0000 4.110E-3 7.9 91.2 1200 13.997 0.0000 3.703E-3 4.4 92.1 1250 15.517 0.0000 9.557E-3 1.0 81.8

FUSE 23.242 0.0000 3.633E-3 0.2 53.8

1.08E-13 1.58E-13 9.21E-14 4.52E-14 3.72E-14 5.44E-14 9.02E-14 1.01E-13 9.60E-14 7.18E-14 3.95E-14 8.93E-15 1.92E-15


JL3-62, Mg-Biotite (J = 0.004085; Sample Weight = 0.1042 g; 36Ar/40Ar Atm = 297.5) 550 10.296 2.284E-2 1.543E-2 16.6 55.7 650 9.916 1.265E-2 9.861E-3 14.7 70.6 750 9.875 1.399E-2 8.917E-3 9.4 73.3 850 9.989 1.300E-2 7.362E-3 5.6 78.2 950 10.043 1.199E-2 8.206E-3 9.0 75.8 1000 10.078 1.301E-2 8.682E-3 7.7 74.5 1050 9.783 8.574E-3 7.182E-3 10.8 78.3 1100 9.784 6.467E-3 6.933E-3 10.9 79.0 1150 10.002 5.355E-3 7.042E-3 7.7 79.1 1200 10.500 9.764E-3 7.995E-3 4.9 77.5 1250 12.109 1.534E-2 1.270E-2 2.1 69.0

FUSE 21.108 7.113E-2 4.963E-2 0.7 30.5

1.63E-13 1.45E-13 9.18E-14 5.45E-14 8.86E-14 7.52E-14 1.06E-13 1.07E-13 7.56E-14 4.80E-14 2.04E-14 6.69E-15


Apparent K/Ca,

mol/mol

24.4 60.0 0.00 0.00

0.00 0.00

0.00 119

66.7 22.3 3.85

25.2 48.1 104

55.1 170 233 177

0.00 482 389 130

46.3 172

0.00 68.3 44.0

35.9 0.00 0.00 0.00 0.00 0.00 0.00 0.00

22.8 41.1 37.2 40.0 43.4 40.0

60.6 80.4 97.1 53.3 33.9 7.31

Apparent Age, Ma

35.3+0.2 35.1+0.2 36.1+0.2 37.4+0.2 37.1+0.2 36.6+0.2 37.1+0.2 36.7+0.2 36.6+0.2 36.0+0.2 31.8+0.7 36.6 36.9+0.3

25.7+2.4 26.0+2.3 24.4+1.3 23.8_+0.3 24.2+0.4 24.2+0.2 24.4+0.1 24.2+0.5 24.1+0.4 25.1+0.3 25.1+0.3 24.5 24.3+0.1

70.1+0.4 86.4+0.4 88.7+0.5 88.1+0.5 83.8+0.5 85.8+0.5 87.6+0.4 88.7+0.5 90.7+0.5 92.0+0.5 94.2+0.5 92.7+1.0 91.4+4.6 86.3

41.8+0.2 50.8+0.3 52.6+0.3 56.7+0.3 55.3+0.3 54.5+0.3 55.6+0.3 56.1+0.3 57.4+0.3 59.0+0.3 60.5+0.5 46.9+1.1

52.7



Temp., 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39Ar, 40Ar, o(2 % of total %

39Ar,

JL-MW3, Mg-Biotite (J = 0.004082; Sample Weight = 0.1036 g; 36Ar/40Ar Atto = 297.5) 550 12.481 1.317E-2 1.629E-2 5.7 61.4 650 11.458 4.575E-3 8.046E-3 7.4 79.2 750 10.790 0.0000 4.121E-3 10.6 88.7 850 10.681 0.0000 3.812E-3 9.1 89.4 900 10.758 0.0000 4.051E-3 6.8 88.8 950 10.754 0.0000 3.855E-3 6.3 89.4 1000 10.386 0.0000 3.364E-3 9.0 90.4 1050 10.312 0.0000 2.906E-3 12.8 91.6 1100 10.436 0.0000 2.692E-3 11.8 92.3 1150 10.607 0.0000 2.760E-3 8.4 92.3 1200 10.777 0.0000 2.881E-3 7.8 92.0 1250 11.247 1.247E-2 3.861E-3 3.2 89.8 1300 12.649 5.076E-2 1.159E-2 0.9 72.9 Fuse 18.615 0.0000 3.981E-2 0.3 36.8

Apparent K/Ca,

mol/mol

JL-Cm, Muscovite (J = 0.007170; Sample Weight = 0.1532 g; 36Ar/40Ar Atto 400 10.017 8.875E-2 2.923E-2 0.3 13.8 475 4.787 1.181E-1 5.753E-3 1.3 64.6 550 5.543 3.642E-1 6.576E-3 2.7 65.3 650 4.504 2.182E-2 2.230E-3 2.9 85.3 850 4.578 7.052E-3 1.432E-3 12.0 90.6 950 4.917 3.729E-3 2.071E-3 24.4 87.4 1050 4.797 4.442E-3 1.653E-3 27.5 89.7 1150 5.205 6.073E-3 2.567E-3 22.0 85.3

FUSE 7.286 2.089E-2 9.679E-3 6.8 60.7

6.08E-14 39.5 7.89E-14 114 1.13E-13 0.00 9.68E-14 0.00 7.18E-14 0.00 6.68E-14 0.00 9.62E-14 0.00 1.37E-13 0.00 1.25E-13 0.00 8.89E-14 0.00 8.31E-14 0.00 3.41E-14 41.7 9.33E-15 10.2 2.68E-15 0.00


= 296.5) 9.65E-15 5.86 4.22E-14 4.40 8.57E-14 1.43 9.25E-14 23.8 3.86E-13 73.7 7.81E-13 139 8.18E-13 117 7.06E-13 8.56 2.19E-13 24.9


JL-Cpi, Muscovite (J = 0.007160; Sample Weight = 0.1025 g; 36Ar/40Ar Atto = 296.5) 2.97E-14 5.55E-14 8.21E-14 1.06E-13

6.62E-13 3.66E-13 6.06E-13 1.86E-13

Total gas No plateau

500 5.831 4.685E-2 1.155E-2 1.4 41.4 600 5.618 1.147E-2 7.111E-3 2.7 62.5 700 4.913 5.092E-3 4.329E-3 3.9 73.8 800 4.416 3.836E-3 2.197E-3 5.0 85.2 950 4.622 1.316E-3 2.068E-3 31.6 86.7 1050 4.679 9.752E-4 2.605E-3 17.5 83.4 1150 4.723 7.545E-4 2.381E-3 29.0 85.0

FUSE 6.502 2.330E-3 7.986E-3 8.9 63.6

= 297.5) 3.38E-14 3.75E-14 5.10E-14 8.24E-14 1.97E-13 1.64E-13 1.24E-13 1.23E-13 2.53E-13 1.10E-13 2.50E-14 6.37E-15


JL2-31, Muscovite (J = 0.004073; Sample Weight = 0.1029 g; 36Ar/40Ar Atm 550 8.841 0.000 2.108E-2 2.8 29.5 650 5.395 0.000 8.766E-3 3.1 51.9 750 4.863 0.000 5.923E-3 4.2 63.9 850 5.561 0.000 8.479E-3 6.8 54.8 900 5.880 0.000 8.747E-3 16.3 55.9 950 5.282 0.000 7.150E-3 13.6 59.9 1000 5.367 0.000 7.156E-3 10.3 60.5 1050 5.445 0.000 7.488E-3 10.2 59.3 1150 4.894 0.000 5.278E-3 21.0 68.0 1250 4.524 0.000 4.204E-3 9.1 72.4 1350 4.875 0.000 6.276E-3 2.1 61.8

FUSE 7.151 0.000 1.703E-2 0.5 29.6

Apparent Age, Ma

55.6+0.3 65.6+0.3 69.1+0.4 69.0+0.4 69.0+0.4 69.4+0.4 67.8+0.4 68.3+0.4 69.6+0.4 70.7+0.4 71.6+0.4 72.9+0.4 66.7+0.7 49.7+1.8 68.3

17.8+_0.4 39.5+_0.2 46.3+0.2 49.0+_0.2 52.9+_0.3 54.8+_0.3 54.8+_0.3 56.6+0.3 56.3+0.3 54.4

11.1

45.3 102 136 395 533 689 223

30.9+_1.7 44.8+0.2 46.3+0.2 47.9+_0.2 51.0+0.3 49.7+0.3 51.1+0.3 52.7+0.3 50.2

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00

0.00 0.00 0.00

19.1+0.2 20.5+0.2 22.7+_0.1 22.3+0.1 24.0+0.2 23.1+_0.1 23.7+0.1 23.6+_0.2 24.3+_0.1 23.9+_0.1 22.0+_0.3 15.5_+0.8 23.4


Temp., 40Ar/39Ar 37Ar/39Ar o C


36Ar/39Ar 39Ar, 40Ar, % of total %

39Ar, Apparent Apparent mol K/Ca, Age,

mol/mol Ma

MM-41, Muscovite (J = 0.003791; Sample Weight = 0.1010 g; 36Ar/40Ar Atm = 295.5) 550 10.410 8.999E-3 2.514E-2 2.7 28.6 2.83E-13 650 6.699 4.697E-3 9.758E-3 3.0 57.0 3.10E-13 750 6.123 0.000 8.013E-3 5.3 61.3 5.54E-13 850 5.631 1.237E-3 6.823E-3 7.6 64.2 7.93E-13 950 5.252 1.526E-3 5.280E-3 20.2 70.3 2.10E-12

1000 4.998 3.257E-3 4.419E-3 11.4 73.9 1.18E-12 1050 4.853 1.865E-3 4.329E-3 8.6 73.6 8.89E-13 1100 4.867 1.096E-3 4.365E-3 9.0 73.5 9.34E-13 1150 4.684 1.699E-3 3.699E-3 11.7 76.7 1.22E-12 1250 4.470 1.676E-3 2.404E-3 12.7 84.1 1.32E-12

FUSE 4.516 2.361E-3 2.637E-3 7.7 82.8 8.03E-13


JL2-104, Muscovite (J = 0.0004187; Sample Weight = 0.1073 g; 36Ar/40Ar Atm = 297.5) 550 9.861 0.000 1.573E-2 3.0 52.8 3.93E-14 650 8.071 0.000 6.599E-3 3.6 75.8 4.78E-14 750 8.186 0.000 5.394E-3 6.1 80.5 8.03E-14 850 8.705 0.000 4.780E-3 13.2 83.7 1.75E-13 900 8.474 0.000 3.703E-3 18.1 87.0 2.40E-13 950 8.368 0.000 3.668E-3 13.8 87.0 1.83E-13

1000 8.486 0.000 3.515E-3 11.3 87.7 1.50E- 13 1050 8.407 0.000 2.787E-3 13.6 90.1 1.81E-13 1100 8.376 0.000 1.861E-3 13.6 93.4 1.81E-13 1150 8.717 0.000 3.830E-3 3.2 86.9 4.24E- 14

FUSE 12.094 0.000 1.773E-2 0.4 56.6 5.58E-15


JLl-148A, Muscovite (J = 0.004174; Sample Weight = 0.1039 g; 36Ar/40Ar Atm = 297.5) 550 7.779 0.000 1.147E-2 2.2 56.4 2.54E-14 650 6.594 0.000 8.214E-3 2.5 63.1 2.89E-14 750 6.059 0.000 4.323E-3 5.0 78.8 5.79E-14 850 6.266 0.000 3.029E-3 9.4 85.6 1.10E-13 900 6.451 0.000 2.478E-3 13.7 88.6 1.60E-13 950 6.476 0.000 2.110E-3 15.5 90.3 1.81E-13

1000 6.472 0.000 2.405E-3 11.7 88.9 1.37E-13 1150 6.564 0.000 1.345E-3 34.0 93.9 3.98E-13 1250 6.988 0.000 3.808E-3 5.3 83.8 6.19E-14

FUSE 9.464 4.705E-2 1.412E-2 0.8 55.9 9.01E-15


JL2-79, Muscovite (J = 0.0004048; Sample Weight = 0.0940 g; 36Ar/40Ar Atm = 297.5) 550 10.647 0.000 2.054E-2 2.1 42.9 650 8.651 0.000 1.116E-2 2.2 61.8 750 8.139 0.000 9.709E-3 3.4 64.7 850 8.791 0.000 9.585E-3 7.3 67.7 900 8.813 0.000 8.185E-3 14.8 72.5 950 8.255 0.000 6.731E-3 14.9 75.8

1000 8.144 0.000 7.004E-3 9.3 75.5 1050 8.321 0.000 8.131E-3 7.5 71.1 1100 8.266 0.000 7.285E-3 7.9 73.9 1150 7.887 0.000 5.258E-3 12.0 80.2 1200 7.552 0.000 3.824E-3 11.8 85.0 1250 7.493 0.000 4.319E-3 4.6 82.9 1300 7.795 0.000 6.400E-3 1.6 75.7

FUSE 8.504 0.000 1.332E-2 0.7 53.7

2.19E-14 2.27E-14 3.55E-14 7.67E-14 1.56E-13 1.56E-13 9.76E-14 7.88E-14 8.36E-14 1.27E-13 1.24E-13 4.86E-14 1.65E-14

7.73E-15


57.8 20.3_+0.6 110.7 25.9_+1.1 0.00 25.5_+0.9 420.5 24.6_+0.6 345.4 25.1_+0.2 159.7 25.1_+0.3 278.8 24.3_+0.8 474.6 24.3_+0.6 306.1 24.4_+0.9 310.3 25.5_+0.4 220.2 25.4_+1.0

24.8 25.1_+0.2

0.00 38.9_+0.3 0.00 45.6_+0.3 0.00 49.1_+0.3 0.00 54.2_+0.3 0.00 54.9_+0.3 0.00 54.2_+0.3 0.00 55.4_+0.3 0.00 56.4_+0.3 0.00 58.1_+0.3 0.00 56.4_+0.3 0.00 51.0_+1.2

54.3

0.00 32.7_+0.2 0.00 31.2_+0.3 0.00 35.6_+0.2 0.00 40.0_+0.2 0.00 42.5_+0.2 0.00 43.5_+0.2 0.00 42.8_+0.2 0.00 45.8_+0.2 0.00 43.6_+0.2 11.1 39.4_+0.8

42.8

0.00 33.1_+0.4 0.00 38.6_+0.3 0.00 38.0_+0.3 0.00 43.0_+0.3 0.00 46.1_+0.3 0.00 45.1_+0.3 0.00 43.8+0.2 0.00 42.7_+0.2 0.00 44.1_+0.2 0.00 45.6_+0.2 0.00 46.3_+0.3 0.00 44.8_+0.3 0.00 42.6_+0.3 0.00 33.0_+0.8

44.1



Temp., 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39Ar, 40Ar, 39Ar, Apparent ø(2 % of total % mol K/Ca,

mol/mol

JL2-92, Muscovite (J = 0.0004163; Sample Weight = 0.1293 g; 36Ar/40Ar Atm = 297.5) 550 6.008 0.000 1.105E-2 3.1 45.6 4.14E-14 0.00 650 5.701 0.000 8.121E-3 3.6 57.8 4.77E-14 0.00 750 5.334 0.000 5.339E-3 5.0 70.3 6.57E-14 0.00 850 5.757 0.000 5.659E-3 6.2 70.8 8.15E-14 0.00 900 6.141 0.000 5.829E-3 8.7 71.9 1.15E-13 0.00 950 6.086 0.000 5.246E-3 11.4 74.4 1.51E-13 0.00 1000 5.874 0.000 4.382E-3 9.9 77.9 1.31E-13 0.00 1050 5.689 0.000 4.234E-3 9.6 77.9 1.27E-13 0.00 1100 5.620 0.000 3.900E-3 10.1 79.4 1.33E-13 0.00 1250 5.758 0.000 2.878E-3 24.6 85.1 3.25E-13 0.00

FUSE 5.801 0.000 3.154E-3 7.8 83.8 1.03E-13 0.00


JL2-89, Muscovite (J = 0.004123; Sample Weight = 0.1128 g; 36Ar/40Ar Atm = 297.5) 550 6.870 4.143E-2 1.408E-2 2.8 39.4 3.67E-14 12.5 650 4.652 7.657E-3 4.697E-3 4.6 70.1 6.07E-14 67.9 750 4.363 4.643E-3 3.904E-3 6.3 73.4 8.32E-14 112 850 4.509 0.000 3.911E-3 7.1 74.2 9.29E-14 0.00 900 4.813 0.000 3.929E-3 10.0 75.8 1.32E-13 0.00 950 4.848 0.000 3.300E-3 9.7 79.8 1.28E-13 0.00 1000 4.788 0.000 2.914E-3 7.9 81.9 1.04E-13 0.00 1050 4.739 0.000 3.676E-3 7.8 77.0 1.02E-13 0.00 1150 4.723 0.000 3.457E-3 17.9 78.2 2.34E-13 0.00 1250 5.003 0.000 2.654E-3 20.7 84.2 2.71E-13 0.00

FUSE 5.477 0.000 3.956E-3 5.1 78.5 6.67E-14 0.00


JLl-161, Muscovite (J = 0.004187; Sample Weight = 0.1252 g; 36Ar/40Ar Atm = 297.5) 550 5.566 0.000 1.058E-2 2.6 43.7 3.86E-14 0.00 650 3.853 0.000 3.472E-3 4.7 73.2 6.96E-14 0.00 750 4.044 0.000 2.691E-3 9.1 80.2 1.35E-13 0.00 850 4.354 0.000 2.818E-3 13.2 80.7 1.97E-13 0.00 900 4.379 0.000 2.035E-3 22.1 86.1 3.28E-13 0.00 950 4.238 0.000 2.487E-3 13.5 82.5 2.00E-13 0.00 1000 4.376 0.000 2.436E-3 11.8 83.4 1.75E-13 0.00 1050 4.400 0.000 1.788E-3 10.6 87.9 1.58E-13 0.00 1100 4.526 0.000 1.073E-3 10.6 92.9 1.58E-13 0.00 1150 5.114 0.000 5.137E-3 1.6 70.2 2.40E-14 0.00

FUSE 17.918 2.173E-1 5.649E-2 0.3 6.9 4.25E-15 1.92


Apparent Age, Ma

20.4+0.2 24.6+0.2

28.0+0.2 30.4+0.2 32.8+0.2 33.7+0.2

34.0+0.2 33.0+0.2 33.2+0.2 36.4+0.2 36.2+0.2 33.2

20.0+0.2 24.1+0.2 23.7+0.1

24.'7_+0.1 26.9+0.2 28.5+0.2 28.9+0.2 26.9+0.2 27.3+0.2 31.1+0.2 31.7+0.2 27.7

18.3+0.2 21.2+0.1 24.3+0.1

26.4+0.1 28.3+0.2 26.2+0.2 27.4_+0.2 29.0+0.2 31.5+0.2 26.9+0.3

9.34+2.0 27.0

JL1-222A, Muscovite (J = 0.005618; Sample Weight = 0.1074 g; 36Ar/40Ar Atm = 296.5) 450 4.376 1.061E-2 7.974E-3 4.1 46.0 5.05E-13 49.0 550 3.870 4.990E-3 6.192E-3 5.6 52.6 7.03E-13 104 650 2.813 4.263E-3 2.448E-3 7.6 74.1 9.47E-13 122 750 3.013 2.459E-3 2.413E-3 10.8 75.2 1.35E-12 211 850 3.014 1.680E-3 1.939E-3 19.3 80.8 2.40E-12 309 950 2.879 1.067E-2 1.812E-3 26.6 81.2 3.32E-12 48.7 1000 3.344 1.559E-2 2.960E-3 8.3 73.7 1.03E-12 33.4 1050 3.481 1.092E-2 2.977E-3 7.1 74.6 8.86E-13 47.6 1150 3.362 9.143E-3 2.460E-3 6.8 78.2 8.48E-13 56.9

FUSE 3.657 5.381E-2 3.085E-3 3.7 75.0 4.66E-13 9.66


20.3+0.1 20.5+0.1 21.0+0.1 22.8+0.1 24.5+0.1 23.6+0.1 24.8+0.1 26.1+0.1 26.5+0.1 27.6+0.1

23.8



Temp., 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39Ar, 40Ar, ø(2 % of total %

39Ar,

JL1-219, Muscovite (J = 0.005815; Sample Weight = 0.1059 g; 36Ar/40Ar Atto = 296.5) 550 5.759 1.148E-2 1.376E-2 1.6 29.3 2.22E-13 650 5.566 7.783E-3 1.192E-2 2.6 36.6 3.51E-13 750 5.595 5.837E-3 1.192E-2 3.0 37.0 4.10E-13 850 4.772 6.866E-3 7.956E-3 5.9 50.6 7.97E-13 950 3.783 2.663E-3 3.785E-3 39.6 70.3 5.37E-12 1000 3.558 3.632E-3 3.562E-3 16.1 70.3 2.19E-12 1100 3.852 4.638E-3 4.884E-3 10.1 62.4 1.37E-12

FUSE 3.497 5.113E-3 3.258E-3 21.1 72.3 2.87E-12


JL1-214A, Muscovite (J = 0.005643; Sample Weight = 0.1027 g; 36Ar/40Ar Atm = 296.5) 450 6.894 3.149E- 1 1.611E-2 1.7 31.2 2.40E- 13 550 7.634 2.418E-1 1.832E-2 2.6 29.2 3.62E-13 650 5.272 1.962E-2 1.087E-2 2.7 39.0 3.76E-13 750 4.111 3.049E-3 6.469E-3 4.8 53.4 6.74E-13 850 4.077 2.726e-3 5.050E-3 11.7 63.3 1.66E-12 950 3.610 9.148E-3 3.331E-3 37.1 72.6 5.23E-12

1000 3.694 1.769E-2 3.828E-3 13.7 69.3 1.93E-12 1050 4.056 1.059E-2 5.385E-3 7.1 60.6 1.01E-12

FUSE 3.667 1.173E-2 3.604E-3 18.6 70.8 2.63E-12


Apparent K/Ca,

mol/mol

45.3 66.8 89.1

75.7

195 143 112

102

1.65 2.15 26.5 171

191 56.8 29.4 49.1

44.3

Apparent Age, Ma

17.6_+0.2

21.3_+0.2 21.6_+0.2 25.2_+0.1 27.7+_0.1 26.0_+0.1 25.0_+0.1 26.3_+0.1 26.2

21.8_+0.2 22.6_+0.2 20.8_+0.1 22.2_+0.1 26.1_+0.1 26.5_+0.1 25.9_+0.1 24.9_+0.1 26.3_+0.1 25.7

JL1-211, Muscovite (J = 0.005754; Sample Weight = 0.1079 g; 36Ar/40Ar Atto = 296.5) 450 5.124 2.856E-3 1.076E-2 3.8 37.8 550 3.857 1.142E-3 5.836E-3 6.3 55.1 650 3.978 1.228E-4 5.587E-3 18.3 58.3 750 3.109 3.742E-4 2.745E-3 22.8 73.7 850 3.212 3.818E-3 3.159E-3 12.6 70.8 950 3.760 2.484E-3 4.992E-3 10.0 60.6

1000 3.875 8.848E-4 5.384E-3 5.6 58.8 1050 3.665 8.992E-4 4.555E-3 6.8 63.1 1150 3.371 1.733E-3 3.500E-3 7.6 69.1 1300 3.100 8.431E-3 2.599E-3 5.7 75.1

FUSE 7.015 1.525E-1 1.539E-2 0.5 35.3

5.77E-13 9.66E-13 2.80E-12 3.50E-12 1.94E-12 1.53E-12 8.57E-13 1.04E-12 1.17E-12 8.76E-13 7.94E-14


SR-82-1, Muscovite (J = 0.006253; Sample Weight = 0.0994 g; 36Ar/40Ar Atm = 294.0) 550 21.490 2.808E-1 6.567E-2 0.2 9.8 650 9.839 6.752E-2 2.815E-2 0.6 15.5 750 5.339 1.357E-2 1.191E-2 1.7 34.0 850 5.355 5.803E-3 1.208E-2 2.9 33.2 900 4.504 4.130E-3 9.016E-3 7.3 40.7 950 3.553 2.168E-3 5.282E-3 14.9 55.9 975 3.410 0.000 4.873E-3 16.3 57.6

1025 3.932 1.869E-3 6.803E-3 19.8 48.7 1075 7.248 0.000 1.816E-2 11.4 25.9 1150 6.893 0.000 1.660E-2 12.1 28.7

FUSE 14.236 0.000 4.150E-2 12.8 13.8

1.12E-14 2.57E-14 7.85E-14 1.31E-13 3.29E-13 6.74E-13 7.36E-13 8.97E-13 5.17E-13 5.47E-13 5.80E-13


182 455

4230 1390

136 209 588

578 300

61.7 3.41

1.85 7.70 38.3 89.6

126 240 0.00 278 0.00

0.00 0.00

20.0_+0.1 21.9_+0.1 23.9_+0.1 23.6_+0.1 23.4_+0.1 23.5_+0.1 23.5_+0.1 23.9_+0.1 24.0_+0.1 24.0_+0.1

25.5_+0.2 23.5

23.5_+2.6 17.1_+0.4 20.3_+0.3 20.0_+0.2 20.6_+0.1 22.3_+0.2 22.0_+0.1 21.5_+0.1

21.0_+0.2 22.2_+0.2 22.1_+0.2 21.9



Temp., 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39At, 40At, 39At, Apparent ø(2 % of total % mol K/Ca,

mol/mol

MW-DP, Biotite (J = 0.003726; Sample Weight = 0.1270 g; 36Ar/40Ar Atm= 295.5) 550 6.009 8.498E-3 7.403E-3 13.9 63.6 1.571E-12 61.2 650 4.862 7.519E-3 3.320E-3 13.8 79.8 1.568E-12 69.1 750 4.575 4.833E-3 1.872E-3 11.5 87.9 1.301E-12 107.6 850 4.631 9.305E-3 2.178E-3 8.1 86.1 9.220E-13 55.9 950 4.687 1.254E-2 2.456E-3 11.6 84.5 1.318E- 12 41.5 1000 4.634 1.513E-2 2.390E-3 10.0 84.8 1.129E-12 34.4 1050 4.591 2.214E-2 1.807E-3 10.9 88.4 1.231E-12 23.5 1150 4.660 3.445E-2 1.804E-3 10.1 88.6 1.140E-12 15.1 1250 4.821 7.378E-2 2.523E-3 7.5 84.5 8.490E-13 7.1

FUSE 5.718 1.445E-1 7.509E-3 2.7 61.2 3.087E-13 3.6


ELM-I, Biotite (J = 0.006446; Sample Weight = 0.2546 g; 36Ar/40Ar Atm= 296.9) 550 10.020 1.633E-2 2.531E-2 0.4 25.3 1.57E-13 750 4.004 9.622E-3 6.846E-3 1.5 49.3 6.37E-13 850 2.657 3.097E-3 1.829E-3 8.1 79.4 3.55E-12 950 2.384 2.781E-3 8.846E-4 27.1 88.8 1.19E-11 1000 2.544 7.363E-3 1.375E-3 16.3 83.8 7.16E-12 1050 2.722 8.325E-3 2.002E-3 12.8 78.1 5.61E-12

FUSE 2.610 1.392E-2 1.645E-3 33.9 81.2 1.49E-11


ELM-3, Biotite (J = 0.006231; Sample Weight = 0.2273 g; 36Ar/40Ar Atm= 294.7) 550 31.634 5.554E-2 1.021E-01 0.5 4.7 9.37E-14 750 6.923 9.794E-2 1.802E-2 1.3 23.1 2.57E-13 850 3.573 2.796E-2 5.525E-3 4.7 54.2 9.50E-13 950 2.601 3.055E-3 1.643E-3 28.8 81.1 5.80E-12 1000 2.607 3.183E-3 1.699E-3 34.8 80.5 7.00E-12 1050 3.287 5.319E-3 3.846E-3 18.0 65.3 3.62E-12

FUSE 4.087 1.310E-2 6.694E-3 12.0 51.5 2.41E-12


*Read 1.875E-2 as 0.01875.

Apparent Age, Ma

25.5+0.4 25.9+0.3 26.8_+0.5 26.6+0.4 26.4+0.3 26.2-+0.4 27.1+0.3 27.5-+0.2 27.2+0.2 23.4-+1.1

26.4

31.8 29.3-+0.5 54.0 22.8+0.2 !68 24.4+0.1

187 24.5+0.1 70.6 24.6+0.1 62.5 24.6+0.1 37.4 24.5-+0.1

24.5 24.5+0.1

9.36 16.5+0.7 5.31 17.9-+0.3 18.6 21.6+0.1 170 23.6-+0.1 163 23.4+0.1

97.8 24.0+0.2 39.7 23.5+0.1

23.4

23.5+0.2

in the Middle Cambrian marbles, and JL-Cpi is a medium- gr•tined schist from the Pioche Shale. The spectra from both of these muscovite samples yield disturbed patterns most easily interpreted as representing argon loss; high temperature heating increment ages are 52.7 and 56.6 Ma, and low temperature heating increment ages are 30.9 to 39.5 Ma, respectively (Figure 4; Plate 1). Sample JL2-31 is a garnet-bearing, medium- to coarse-grained schist from the base of the Pioche Shale from Smith Creek on the east flank of the range. This sample contains a subhorizontal mylonitic foliation and a WNW-ESE trending mineral elongation lineation. This sample yields an age spectrum with minor argon loss and a minimum low temperature heating increment age of 19.1 Ma. The middle part of the spectrum (80% of the 39At released) yields a near plateau at 23.4 Ma (Figure 4; Plate 1).

Muscovite samples JL2-104, JLl-148A, JL2-79, JL2-92, JL2-89, and JL 1-161 form a west-to-east transect in the southern part of the range (Figure 5; Plate 1). All of these

samples exhibit a well-developed subhorizontal mylonitic foliation and WNW-ESE trending mineral elongation lineation; the strain associated with this fabric increases eastward. Age spectra from these samples are variably disturbed. Total gas ages decrease from west to east with a high of 54.3 Ma on the west and a low of 27.0 Ma on the east. The oldest ages for high temperature heating increments and the youngest ages for low temperature heating increments also decrease from west to east. On the west flank of the range the oldest ages for high temperature heating increments are 45.6 to 58.1 Ma and the youngest ages for low temperature heating increments are 31.1 to 38.9 Ma, whereas on the east flank they are 31.5 to 36.4 Ma and 18.3 to 20.4 Ma, respectively.

There are at least two possible interpretations of the disturbed muscovite age spectra from these two transects across the northern Snake Range. On the west flank of the range the high-temperature ages of 46 to 58 Ma may represent minimum age estimates of muscovite closure to argon at about 325øC for


rocks that reached • metamorphic temperatures during a prior event, probably during the Late Cretaceous. The accompanying low-temperature ages of 31-40 Ma may be the result of a short-lived, younger thermal event at temperatures <325øC that partially degassed these Late Cretaceous muscovites, thus representing maximum age estimates for this younger thermal event and the mylonitic foliafion. On the northeast flank of the range a near-plateau age of 23 Ma for sample JL2-31 suggests that this is the only sample at the Prospect Mountain Quartzite-Pioche Shale horizon which underwent sustained heating at >325øC during this younger event and, as such, is a cooling age and gives a minimum age estimate for the time of mylonific deformation. The high- temperature ages of 32-36 Ma on the southeast flank of the range are maximum age estimates for low greenschist facies metamorphism and mylonific deformation during extension and may represent a more sustained thermal event at <325øC than on the west flank of the range. The low-temperature ages (18- 20 Ma) from these samples indicate either a maximum age of yet a younger thermal event that partially degassed these muscovites or the time of final closure to argon at about 270øC following slow cooling [Snee et al., 1988]. An alternative interpretation (or an additional interpretation) for these disturbed age spectra is that the spectra from the west flank of the range may be recording degassing from two muscovite populations. The low temperature heating increment ages could reflect degassing of new fine-grained white mica that grew during mylonific deformation at temperatures <325øC during the Tertiary. The degassing profile of these Tertiary muscovites may be superimposed on the degassing profile of older, medium-grained muscovites of Late Cretaceous age. The resultant age spectrum would be a disturbed pattern similar to what is actually measured [cf. Wijbrans and McDougall, 1986]. In this interpretation the eastward decrease in ages reflects increasing temperatures associated with mylonitic deformation that nearly to completely resets the K-Ar system in the older muscovites.

Samples from two vertical transects of lower plate rocks were collected to characterize the vertical thermal profile during mylonitic deformation and the timing of mylonitic deformation. Both transects are located on the east flank of the

range where canyons deeply incise lower plate rocks providing excellent structural relief (i.e., a composite structural relief of up to -1.5 km) (Figures 6 and 7; Plate 1). Samples JL-MW3, MW-DP, JL2-31, MM-41, and MM-30 are from a transect in the northeastern part of the range (Figure 6; Plate 1). Sample JL-MW3, described above, is from the Dunderberg Shale and yields a disturbed spectrum indicating a reheating event at <57 Ma and minimum age estimate for peak metamorphism of 73 Ma. Biotite sample MW-DP was collected from a dacite porphyry dike that intrudes into the upper part of the Middle Cambrian marbles and calc-schists on the northeast flank of the range (Figure 6; Plate 1). This dike has been penetratively deformed and boudinaged by the ductile thinning and stretching deformation. This sample yields a slightly disturbed spectrum with a TG age of 26.4 Ma. Sample JL2-31, described above, is from the Pioche Shale and yields a near plateau age of 23.4 Ma. Below this is sample MM-41, a coarse-grained garnet schist within the Osceola argillite. This sample contains a well- developed subhorizontal mylonitic foliation and WNW-ESE trending mineral elongation lineation. Metamorphic temperatures during this deformation were sufficient for new, coarse-grained growth of biotite and muscovite around older, relict Late Cretaceous garnet and staurolite. The age spectrum from this muscovite yields a WAP age of 25.1 + 0.2 Ma (Figure 6; Plate 1). Muscovite and orthoclase, from sample

MM-30, are from a small two-mica granite sill exposed in the deepest part of the Smith Creek area (Figure 6; Plate 1). Cross-cutting relationships indicate that this sill is younger than 82 Ma year old pegmatites and aplites in this area. The margins of the sill, like the country rocks, are strongly deformed, exhibiting a well-developed subhorizontal mylonitic foliation and WNW-ESE trending mineral elongation lineation, whereas the core of the sill is only weakly deformed. Muscovite from the core yields a WAP age of 24.3 + 0.1 Ma. Orthoclase from this sample yields an excess argon pattern over the first 2.9% of the 39Ar released, followed by a saddle-shaped spectrum with an increase in age from a TMIN of 16.7 Ma to a TMAX of 28.1 Ma (Figure 6; Plate 1). Orthoclases that yield saddle-shaped age spectra are common [Maluski, 1978; Albarede et al., 1978; Berger and York, 1981; Zeitler and FitzGerald, 1986] and have been interpreted to be the result of excess argon [Harrison and McDougall, 1981]. Zeitler and FitzGerald [1986] have demonstrated that the minimum low temperature heating increment ages correlate well to zircon fission track ages, thus indicating that the orthoclase cooled below about 200øC at that time (the estimated fission track annealing temperature for zircons) [Zeitler, 1985] and that the maximum high temperature heating increment ages may be geologically meaningless. Recent apatite fission track analyses, however, from lower plate rocks on the eastern flank of the Snake Range indicate cooling below 60 ø- 120øC at about 17 Ma [Miller et al., 1989]. In light of this, we interpret the minimum low temperature heating increment age as documenting cooling well below 200øC, to as low as 60 ø- 120øC, by about 17 Ma, and the maximum high temperature heating increment age of 28.1 Ma as representing the time of cooling below an unknown, but >325øC, closure temperature.

Muscovite samples JL2-89, JL1-222A, JL1-219, JL1-214A, and JL1-211 were collected along a vertical transect to the deepest exposures of lower plate rocks in Hendry's Creek and have been, for the most part, described by Lee et al. [1987] (Figure 7; Plate 1). All samples are strongly deformed, exhibiting a well-developed mylonitic fabric. The top three samples, JL2-89, JL1-222A, and JL1-219, exhibit similar disturbed argon loss age spectra. Minimum low temperature heating increment ages range from 17.6 Ma to 20.3 Ma, and maximum high temperature heating increment ages range from 27.6 Ma to 31.7 Ma. At the bottom of the transect, samples JL 1-214A and JL 1-211 yield flat age spectra that nearly plateau with TG ages of 25.6 Ma and 23.5 Ma, respectively. We interpret the data from both of these transects as indicating increasing temperatures with depth during ductile extension in the Tertiary. Temperatures at high structural levels were either <325øC or not maintained at >325øC for a long enough time to result in complete degassing of samples at some time younger than 28-32 Ma. Temperatures at the deepest structural levels were >325øC resulting in the near to complete resetting of argon in these muscovites at 24-25 Ma. The youngest ages, 18-20 Ma, for low temperature heating increments represent the time of slow cooling to temperatures below about 270øC.

Along the southern flank of the range, three mica samples yield similar ages to the deepest structural levels along the east flank of the range and have been described by Lee et al. [1987]. Two biotite samples, ELM-1 and ELM-3, both possessing the mylonitic foliation, were collected from the eastern part of the mid-Jurassic tonalite (Plate 1). These samples yield WAP ages of 24.5 + 0.1 Ma and 23.5 + 0.2 Ma, respectively. Muscovite sample SR-82-1, from a pendant of strongly deformed Prospect Mountain Quartzite in the mid-Jurassic tonalite from this same region, yields a near plateau with a TG age of 21.9 Ma. Samples ELM-3 and SR-82-1 exhibit minimum low


114ø23'38 ' 1 14015 '

3903 • .

39ø1:

N

' 44

! : ! ,

o 5

KILOMETERS ;

37 •',• 24 37 24 25

ß

i s s

SOURCES OF DATA

K-AR MIlSCO VlTE

• ARMSTRONG AND HANSEN, 1966

•, LEE ET AL., 1970

I LEE ET AL., 1980

40AR739 AR MUSCOVITE

* LEE ET AL., 1987 • THIS PAPER

.--' CHRONTOUR

.o EXTENSIONAL STRAIN

BOUNDARY

39ø15'

1 14ø15' 114ø2'54 '

Fig. 8. Chrontours of published K/At muscovite ages and 40Ar/39Ar total gas ages from this study, from the lower plate Prospect Mountain Quartzite and Pioche Shale. Chrontour ages decrease monotoncially from west to east in the direction of increasing finite strain and an increasing component of top-to-the-east noncoaxial shear. Note that the areal limit of mylonitic fabrics coincides with the area of partial to complete resetting of the K-At isotopic system in Late Cretaceous muscovites.

temperature heating increment ages of 16.5-17.2 Ma and, as interpreted above, are probably the time of slow cooling to temperatures below about 270øC.

DISCUSSION AND CONCLUSIONS

The 40Ar/39Ar thermochronology data presented in this paper allows constraints to be placed on the timing and thermal conditions of Tertiary metamorphism. In addition, the three- dimensional array of ages obtained provides evidence for steep thermal gradients during Tertiary mylonitic deformation and differential cooling of lower plate rocks.

Timing and Temperature of Mylonitization

Mylonitic deformation in the lower plate can be bracketed between 37 Ma and about 24 Ma. This is based on the fact that muscovite-bearing rhyolite porphyry dikes, which are

pretectonic with respect to mylonitic deformation, yield a muscovite 40Ar/39Ar WAP age of 36.9 Ma and metamorphic muscovites, which define the mylonitic fabric at deeper structural levels within the lower plate, yield plateau or near plateau ages of about 24 Ma.

Based on the flow laws of "wet" quartzite, plastic deformation of quartz is believed to be possible at temperatures as low as about 300øC [Koch et al., 1989], suggesting that deformation was likely ongoing at 24 Ma on the east flank of the range but probably ceased shortly thereafter as rocks cooled through the muscovite blocking temperature. It is interesting to note that the areal limit of mylonitic fabrics in the northern Snake Range coincides with the area of partial to complete resetting of the K- Ar isotopic system in Late Cretaceous muscovites (Figure 8). This suggests that plastic flow of quartz can occur at temperatures close to or just below the blocking temperature of muscovite (i.e., temperatures were at least about 270ø-300øC). Alternatively, if the heating interval was short, temperatures could have been higher than 300øC, as argon diffusion is both temperature and time dependent.


Steep Thermal Gradients

On the east flank of the range, several lines of evidence suggest steep veffical thermal gradients during Tertiary mylonitic deformation. Plastic flow of quartz and calcite, but no new mineral growth, occurs in Upper Cambrian units on the east flank of the range, and magnesium-rich biotite grains from these same rocks (e.g., sample JL-MW3) still preserve evidence for Late Cretaceous metamorphism. These relations suggest that at these structural levels, temperatures much greater than about 300øC either were never attained or were attained for only a short period of time during the Tertiary. In contrast, at slightly deeper structural levels (a present vertical structural distance of about 0.2 km), age spectra document temperatures >325øC during Tertiary mylonitic deformation. At even deeper structural levels (a present vertical structural distance of about 1.0 km), muscovite-biotite geothermometry from the Osceola argillite yields peak metamorphic temperatures of about 460øC during the Tertiary [Huggins, 1990] (Figure 6), suggesting a peak TMAX gradient of about 160øC/km. However, based on textural arguments, deformation continued after the peak of metamorphism, which would effectively "collapse" isograds and result in the apparent steep thermal gradient. A minimum TMAX gradient of about 55øC/km (+15), which is also steep, is suggested if peak temperatures were attained prior to the onset of ductile thinning of lower plate units. The steep thermal gradient suggests contact rather than regional metamorphism during Tertiary extension and mylonitic deformation, but no plutons of demonstrable Oligocene age have been documented at the present levels of exposure in the northern Snake Range, although they may be present at greater depths. Alternatively (or in addition), the steep vertical thermal gradient may have been enhanced as a result of top-to-the-east shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories. Clearly, if peak temperatures were reached at some time after the onset of ductile thinning in the lower plate, the pe• thermal gradient would be higher.

The complex age spectra for muscovite obtained in this study were largely unexpected and indicate that much of the lower plate was only partially reset in the Tertiary, suggesting that temperatures in a broad portion of lower plate rocks did not exceed approximately 325øC during mylonitic deformation. These relations suggest that lower plate rocks did not reside at great depths within the crust unless temperature gradients were very low (i.e., less than about 25øC). This inference is compatible with earlier estimates of depth based on stratigraphic arguments [i.e., Gans and Miller, 1983]. The evidence for steep vertical TMAX gradients also supports the inference that lower plate deformation was not restricted to a shear zone of finite width but represents the upper levels of a region of the crust which has undergone extensive plastic flow at even higher temperatures. At these higher temperatures, rocks will yield continuously by plastic flow beneath supracrustal normal faults or ductile shear zones and perhaps be characterized by an increasing component of pure shear.

Differential Cooling

Although mylonitic deformation may have been ongoing throughout this 13 m.y. period (37 to 24 Ma), the data suggest that deformation was not synchronous throughout the range. On the western flank of the range, temperatures in lower plate rocks were well below about 325øC (the argon blocking temperature for muscovite) by about 30 Ma (minimum age of

age spectrum for microcline sample 16-117) and ductile deformation had ceased, while on the eastern flank of the range, lower plate rocks were still at temperatures above 325øC and, based on textural arguments, still deforming plastically at this time. This is also reflected in the chrontours of published K./Ar muscovite ages [Armstrong and Hansen, 1966; Lee et al., 1970, 1980], along with 40Ar/39Ar TG ages from this study, from the Prospect Mountain Quartzite and Pioche Shale, which montonically decrease in age from west to east (Figure 8). These data provide evidence for a temperature difference within a given structural horizon across the present width of the range (approximately 15 km) from about 30 to 24 Ma. These relations indicate either a long-lived heating event requiring slower cooling to the east or an eastward dip of lower plate layering due to top-to-the-east shear in an east dipping shear zone resulting in differential cooling (asymmetric unroofing) as lower plate rocks passed through a series of subhorizontal isotherms. Alternatively (or in addtion), no data exclude an eastward dip of lower plate units (possibly as much as 20 ø ) [P.B. Gans, personal communication, 1985] prior to the onset of extension, and such a dip would also result in differential cooling of lower plate rocks as they were uplifted through subhorizontal isotherms.

Clearly, the original dip of the detachment fault is linked to which of these different hypotheses best explains these data. The 40Ar/39Ar data alone do not contradict the pure shear model of Miller et al. (1983], whereby the detachment fault originates as a subhorizontal ductile-brittle transition, provided the distribution of cooling ages is the result of a lateral thermal gradient. However, finite strain, mesoscopic and petrofabric data from lower plate rocks indicate an increasing component of simple shear in a down-to-the-southeast shear zone along the eastern half of the range [Lee et al., 1987].

The thermochronologic data alone do not constrain the original dip of the shear zone (e.g., low angle versus high angle). Finite strain and mesoscopic structural arguments, however, suggest that the zone of shear must have dipped at least 40 ø more steeply to the east with respect to lower plate layering [Lee et al., 1987]. In light of this, the 40Ar/39Ar data are compatible with the two-stage model of Lee et al. [ 1987], whereby an early component of coaxial strain is superimposed by a component of top-to-the-east noncoaxial strain related to uplift within a younger, moderate to high-angle shear zone along the eastern half of the range. The thermochronologic data are also compatible with a multiple-fault hypothesis [Buck, 1988], whereby a continuum of superimposed high-angle, planar normal faults propagate from west to east as the nonrigid lower plate rolls over to flat-lying as it is translated up and out from beneath these fault systems. Based on the resolution afforded by the thermochronologic data, a two or three high angle fault hypothesis cannot be distinguished from a continuum fault hypothesis. Both of these models provide a mechanism for earlier uplift and cooling of rocks on the west flank of the range as compared to rocks on the east flank of the range. The pure shear model is the only one that requires lateral thermal gradients.

A progression of cooling ages similar to that documented in the northern Snake Range is observed in the Ruby Mountains metamorphic core complex, where a westward decrease in K/Ar and 40Ar/39Ar biotite and apatite fission track ages occurs in the direction of increasing strain and top-to-the-west simple shear [Kistler et al., 1981; Lister and Snoke 1984; Blackwell et al., 1984; Dallmeyer et al., 1986]. These data pose similar problems. In both cases, three-dimensional arrays of higher resolution thermochronologic data are needed to unequivocally document and understand the cause of this progression and


whether it is due to tilting or unroofing along high-angle or low-angle normal fault systems.

Acknowledgments. Discussions with P.B. Gans and T.A. Little, and especially E.L. Miller, helped clarify the views presented in this manuscript. P.B. Gans helped with the mineral separations and sample analyses. M. Kunk at the USGS maintained the 40Ar/39Ar extraction line. Financial support was provided by NSF grants 8418678 and 8804814 awarded to E.L. Miller. E.L. Miller thoroughly read earlier versions of this manuscript. Art Snoke and an anonymous reviewer provided constructive reviews.

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(Received December 7, 1989; revised August 17, 1990; accepted August 17, 1990.)

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