Geology

doi: 10.1130/0091-7613(1991)019<1213:AODFKA>2.3.CO;2 1991;19;1213-1216Geology

 W. James Dunlap, Christian Teyssier, Ian McDougall and Suzanne Baldwin 

Ar dating of white micas39Ar/40Ages of deformation from K/Ar and   

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Ages of deformation from K/Ar and 40Ar/39Ar dating of white micas

W. James Dunlap, Christian Teyssier Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455

Ian McDougall, Suzanne Baldwin Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia

ABSTRACT A structural and isotopic dating study of white micas from mylonites in the Ruby Gap

duplex of central Australia shows that mylonitic deformation occurred during the Paleozoic Alice Springs orogeny. Deformation of the white micas took place under greenschist facies conditions, in the approximate temperature range 250-350 °C. Two distinct populations of white mica have been found: (1) porphyroclasts of original muscovite and (2) neocrystallized phengites that define the fabric associated with deformation. 4 0Ar/3 9Ar age spectra of neocrys-tallized phengites yield plateau-like spectra of Paleozoic age. In contrast , 4 0AT/ 3 9At age spec-tra of muscovite porphyroclasts exhibit Proterozoic apparent ages, indicating that white micas were not open to significant argon diffusion during formation of the Ruby Gap duplex. We conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system.

INTRODUCTION White micas offer considerable potential for

dating crustal deformation because they are sta-ble under a wide range of metamorphic condi-tions, they commonly define deformation fab-rics, and they are amenable to dating by the K/Ar and 4 0Ar/3 9Ar methods. Successful dat-ing of deformation requires an understanding of the timing of white mica crystallization, defor-mation, or recrystallization relative to fabric de-velopment, and the identification of micas generated by these different processes. The term "recrystallization," as used in this paper, encom-passes static and dynamic recrystallization or neocrystallization (formation of isolated new crystals).

Loss of radiogenic argon from white micas depends upon the temperature attained during deformation, the residence time at elevated temperature, and whether partial or complete recrystallization occurs. Diffusion theory sug-gests that argon will begin to diffuse out of white micas at temperatures above —350 °C (Robbins, 1972; Dodson and McClelland-Brown, 1985), yet below this temperature argon normally will be retained in the lattice quantitatively (Dodson, 1973). Thus, if deformation occurred well above the temperature of argon retention, then it is likely that a measured age of a white mica will record the time since cooling below the closure temperature for argon (Dodson, 1973), rather than the timing of the actual deformation. If deformation of white mica took place at temperatures below that of significant argon loss, a measured age may or may not provide information relating to deformation. Should the white mica remain unrecrystallized, then it

would be expected that little or no resetting of the mica age would occur. On the other hand, if total recrystallization took place, with formation of new mica, then it is likely that a good estimate of the timing of deformation can be obtained. If only partial recrystallization occurs, then a measured K/Ar age or 4 0 Ar/ 3 9 Ar age spectrum probably will reflect the mixed population of micas (e.g., Wijbrans and McDougall, 1986).

There is conflicting evidence in the literature as to the relative importance and roles of ther-mally activated diffusion and recrystallization in setting or resetting K/Ar and 4 0Ar/ 3 9Ar ages of minerals during deformation (Chopin and Maluski, 1980; Kligfield et al., 1986; Wijbrans and McDougall, 1986). In this paper we investi-gate different populations of white micas from a ductile thrust system developed under mid-crustal, greenschist facies conditions in an oro-genic belt in central Australia. Microstructural, chemical, and isotopic age data show that neo-

crystallized white micas provide a means of monitoring and dating the deformation history, and that unrecrystallized white micas are hardly reset during deformation at such shallow levels in the crust.

STRUCTURAL GEOLOGY AND KINEMATIC EVOLUTION

The intracratonic Arltunga nappe complex of central Australia formed during the mid-Paleo-zoic Alice Springs orogeny (Stewart, 1971) when Proterozoic basement gneisses were thrust over sedimentary cover rocks of the Late Prot-erozoic Amadeus Basin (Forman, 1971; Shaw et al., 1971; Teyssier, 1985; Dunlap et al., 1990).

The Ruby Gap duplex is a 3-km-thick thrust system composing the eastern flank of the Arl-tunga nappe complex. Late Proterozoic Heavi-tree Quartzite, the basal member of the Ama-deus Basin, is a distinctive marker horizon that delineates six thrust sheets within the Ruby Gap duplex (Fig. 1). Deformation is concentrated in the Heavitree Quartzite and Bitter Springs do-lomite, and finite strain increases toward the north; the most deformed rocks are in sheet 6 of the duplex. Parautochthonous sheet 1 is weakly deformed except where imbricate thrust faults cut across the unconformity between basement rocks and the Heavitree Quartzite. A north-dipping cleavage of white mica is especially well developed adjacent to the imbricate faults and in close proximity to the thrust fault at the base of sheet 2 (Fig. 1). Sheet 2 consists of a well-foliated quartzite overlain by an imbricate zone

South

Bitter Springs Formation Heavilree Quartzite Basement Gneisses

D u p 1 e *

-10km

Figure 1. North-south cross section of Ruby Gap duplex showing lithologic units, thrust faults, sample numbers (boxes), and thrust-sheet numbers (circles). Sample locations projected onto plane oi cross section. Inset shows ductile duplex formation; dashed line locates incipient thrust.

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containing isoclinally folded quartzite and do-lomite. A prominent north-south-oriented linea-tion is defined by the elongation of quartz and mica grains. Heavitree Quartzite in sheets 3-6 contains a strong mylonitic fabric with foliation parallel to all lithologic contacts. Basement gneisses within these sheets are variably de-formed, have undergone retrograde metamor-phism, and contain a mineral lineation oriented consistently north-south. South-directed imbri-cate thrust faults, south-verging folds, shear bands, asymmetric porphyroclasts, and S-C-C' relations in sheets 1-6 indicate, quite unambig-uously, south-directed shear during duplex formation.

Quartz grains in the Heavitree Quartzite within sheets 1 and 2 have undergone dissolu-tion-reprecipitation, as well as intracrystalline deformation, as evidenced by preferred orienta-tion of grain shape and core and mantle micro-structures. In general, Heavitree Quartzite with-in sheets 3-6 is entirely recrystallized, with microstructures typical of dynamic recrystalliza-tion accommodated by grain-boundary migra-tion (Urai et al., 1986). Recrystallized grain size increases from 80 /urn in sheet 3 to 300 /um in sheet 6. Feldspars in basement gneisses of sheets 3-6 of the duplex exhibit cataclastic textures, indicative of low-temperature deformation, whereas quartz exhibits dynamic recrystalliza-tion microstructures only. Cross-section recon-structions indicate that temperatures during deformation were approximately 250-350 °C assuming a normal geothermal gradient and a 10 km maximum thickness of the overlying nappes.

Formation of the Ruby Gap duplex may have involved piggyback thrusting (Fig. 1), which oc-curs when thrust displacement is transferred to the lowest, newly formed thrust, resulting in the upper thrust sheets being carried in a piggyback fashion (Boyer and Elliot, 1982, Fig. 19). An important characteristic of ductile duplexes formed by piggyback thrusting is that the upper sheets are not carried passively, but continue to deform, accumulating more strain over time (Fischer and Coward, 1982). We interpret the northward-increasing finite strain gradient in the Ruby Gap duplex to reflect a piggyback se-quence of imbrication. However, we cannot rule out the possibility of the reverse sequence of imbrication. In general, cooling ages increase toward the north on the scale of the entire Arl-tunga nappe complex, suggesting a history of piggyback thrusting (Dunlap et al., 1990).

WHITE MICA Three texturally distinct types of white mica

are present in the samples studied: (1) large (0.5-8 mm) porphyroclasts of original musco-vite grains; (2) masses of segmented muscovite grains that are not in optical continuity (Wil-son and Bell, 1979) but resemble porphyroclasts

in outline; and (3) fine (<250 /xm), chemically distinct, neocrystallized white micas that define the mylonitic foliation. Seven samples of quartz-ite mylonites, from sheets 1,3,5, and 6, and two samples of granitic gneiss, from sheets 4 and 5, were selected for microstructural, chemical, and isotopic analysis because they all contain a white mica foliation related to duplex formation. The quartzite mylonites contain type 3 white mica only; however, the least deformed sheet 1 quartz-ite contains both type 1 and type 3 mica. The granitic gneiss contains all three textural types of white mica.

In quartzite sample 731 from sheet 1, the least deformed sheet, fine neocrystallized white mica defines a cleavage, and remnant detrital white mica is present as large isolated flakes (Fig. 2) or as parts of rounded lithic fragments. In sample 520, also from sheet 1, quartz is almost entirely recrystallized, and no detrital white mica ap-pears to remain. Neocrystallized white mica oc-curs as dispersed flakes among the fine recrystal-lized quartz grains. Therefore, it appears that deformation resulted in mechanical disintegra-tion and dissolution of the detrital white mica and an increase in the modal proportion of neo-crystallized white mica. Neocrystallized white micas in samples 764,797, and 533, of quartzite in sheets, 3,5, and 6, respectively, have a strong preferred orientation defining the mylonitic foli-ation, are homogeneously distributed in a matrix of recrystallized quartz, and show very little in-ternal deformation optically.

White micas from granitic gneiss samples 627 and 796 from sheets 4 and 5, respectively, of the duplex contain large type 1 porphyroclasts, segmented type 2 masses of original white mica, and type 3 neocrystallized white mica that par-tially defines the mylonitic foliation. White mica porphyroclasts exhibit openly to tightly folded cleavage planes and may be partially segmented. Type 2 masses exhibit abundant kinks, cleavage cracks, and dissolution surfaces. In contrast, neocrystallized white mica defines foliation and shows very little evidence of internal deforma-tion. In view of the marked textural differences

between the different populations of white mica, we suggest that the porphyroclasts have not un-dergone significant dissolution, whereas the fine-grained white micas have neocrystallized from a fluid phase during deformation. This interpreta-tion is supported by chemical and isotopic anal-yses, as discussed below.

Chemical Analysis An electron microprobe study was conducted

on white micas in Heavitree Quartzite and basement gneisses from sheet 5 and Heavitree Quartzite from sheet 1. White micas from sheet 5 exhibit a bimodal distribution of compositions. White mica porphyroclasts from pegmatite gneiss are muscovites, although some analyses within 5 um of porphyroclast margins plot in the phengite field (Si4+ >3.25). Coexisting neo-crystallized white micas that have grown in por-phyroclast pressure shadows plot exclusively in the phengite field. Newly grown white mica in regrograde hornblende gneiss and neocrystal-lized white micas from sheet 5 quartzite are, without exception, phengites.

White micas from the quartzite of sheet 1 also exhibit bimodal compositions. Old detrital micas are muscovites, whereas neocrystallized white micas that define the foliation associated with deformation are all phengites (Fig. 2). Quartzite mylonites from sheets 3-6 of the du-plex are entirely recrystallized and are not ex-pected to contain any detrital muscovite component.

On the basis of the stability field of phengite micas in the presence of excess water (Velde, 1967), phengites of the Ruby Gap duplex quartz-ites would be stable below 350 °C for a pressure equivalent to 10 km, the estimated maximum thickness of thrust sheets overlying the duplex.

4 0Ar/3 9Ar AND K/Ar RESULTS Neocrystallized phengite from a range of rock

types in the Ruby Gap duplex, muscovite from basement gneisses, and detrital muscovite pre-served in relatively undeformed Heavitree Quartzite have been analyzed by both the K/Ar

Figure 2. Photomicrograph of deformed quartzite sam-ple 731 from narrow shear zone in sheet 1 showing deformed detrital grains of quartz and type 1 white mica. Fine-grained new white mica (type 3) has grown between recrystal-lized quartz grains.

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and 4 0Ar/3 9Ar methods of isotopic dating. Sample locations are shown in Figure 1, and 40Ar/39Ar age spectra are shown in Figure 3. The K/Ar data for the white micas are compiled in Table 1 and grouped according to white mica chemistry. All of the neocrystallized phengites yield Paleozoic K/Ar ages in the range 311 to 413 Ma, whereas the muscovites give K/Ar ages exceeding 1100 Ma, showing that the Alice Springs orogeny did not cause major resetting of these metamorphic and detrital muscovites.

Muscovite porphyroclasts from basement gneiss samples 627 and 796 from sheets 4 and 5 of the duplex, respectively, yield 40Ar/39Ar apparent age spectra (Fig. 3A) that rise to plateau-like parts in the latter stages of the argon release at about 1610 +5 Ma and 1120 ±5 Ma, respectively. In the early stages of gas release, apparent ages as low as 730 ±2 Ma and 522 ±4 Ma are found for samples 627 and 796, respec-tively. Because small amounts of neocrystallized phengite occur in the cleavages of the muscovite

porphyroclasts, the form of the age spectra is probably at least partly the result of the early outgassing of the young and fine-grained phen-gite, yielding mixed age spectra. However, we cannot rule out the possibility of minor resetting of the muscovite porphyroclasts by diffusive loss of 40Ar, such as that found by Scaillet et al. (1990).

Concentrates of neocrystallized phengite also were made from gneiss samples 627 and 796, and these yielded K/Ar ages of 414 ±4 Ma and 340 ±4 Ma, respectively. Contamination of the phengites by porphyroclast muscovite means that even the younger age is likely to be a maxi-mum for the phengites in these samples.

Large detrital muscovite flakes extracted from mildly deformed quartzite 731 from sheet 1 of the duplex (Figs. 1 and 2) yielded a K/Ar age of 1569 ±16 Ma, suggesting that the quartzite formed from erosion of Middle Proterozoic rocks. Although contaminated by up to 5% detrital muscovite, the fine-grained neocrys-

tallized phengite yielded a K/Ar age of 376 Ma, indicating a clear association with Paleozoic deformation.

Total fusion 4 0Ar/3 9Ar (and K/Ar) ages for the pure phengites from Heavitree Quartzite mylonites of sheets 1, 3, 5, and 6 of the Ruby Gap duplex show a systematic decrease in ap-parent age from 328 to 311 Ma toward the northern, more ductile part of the duplex (Table 1, Fig. 1). The progression of age also is evident in the age spectra (Fig. 3B).

Phengite 533 (sheet 6) yielded a plateau age of 311 ±1 Ma for >80% of the gas release, with high apparent ages in the low-temperature part of the age spectrum. This phengite is fresh and undeformed, and we tentatively interpret the age as that of neocrystallization.

Phengite from sample 797 of sheet 5 yields an age spectrum that rises over a narrow range of age from 305 to 322 Ma, disregarding the final 1.8% of gas release that yields an age of 426 Ma. The flattish part of the spectrum between 24% and 64% of 39Ar gas release yields an age of 314 ±1 Ma. The phengite from sample 764 from quartzite in sheet 3 of the duplex is virtually flat over more than 90% of the gas release; its calcu-lated age is 320 ±1 Ma. Phengites of both sam-ples 797 and 764 are free of deformation, and we suggest that their age may approximate that of neocrystallization during deformation.

The phengite from sample 520 from sheet 1 yields a saddle-shaped spectrum with a low age, 273 Ma, for the first 5% of gas release. The second step (650 °C) yields an age of 340 Ma, and the ages for successive steps decrease to 325 Ma before rising again to 345 Ma in the last 9% of gas release. The phengite concentrate consists of a mixture of deformed and undeformed crys-tals that appear to be chemically identical. The possibility exists that this age spectrum reflects neocrystallization of phengite over an extended period, but we are unable to prove that this is the case.

DISCUSSION We have documented variations in micro-

structure, strain, metamorphism, white mica chemistry, and 40Ar/39Ar ages that are related to the map-scale geometric framework of the Ruby Gap duplex. K/Ar and 40Ar/39Ar data suggest that the Ruby Gap duplex was not above the temperature for significant argon loss from muscovite for any extended period of time. Below we explore the possibility that the 4 0Ar/3 9Ar ages of white micas may record neo-crystallization during deformation at relatively low temperatures.

Effect of Deformation on White Micas Deformation of white micas under green-

schist facies conditions generally results in a re-duction of grain size and dissolution of old grains as well as neocrystallization of new

1 8 0 0 1

1 6 0 0 "

f 1 14 00""

3 6 0

5 5 1 2 0 0 " CS a 10 00 <

8 0 0

6 0 0 +

627

796

400") 1 ' 1 1 1 1 ' 1 1 1 0.0 0.2 0.4 0.6 0 .8 1.0

A F r a c t i o n 3 9 Ar Re leased

2 6 0 0.0 0 .2 0.4 0.6 0 .8 1.0

F r a c t i o n 3 9 Ar Re leased

Figure 3. *°Ar/39Ar incremental release diagrams for muscovite and phengite from Ruby Gap duplex. A: Age spectra from muscovite porphyroclasts 627 and 796 from sheets 4 and 5, respec-tively. B: Age spectra for phengite from quartzite mylonites 520,764, 797, and 533 of sheets 1,3, 5, and 6, respectively. Note that data were lost (or one fraction in step-heating experiment on sample 797.

TABLE 1. ANALYTICAL DATA FOR WHITE MICAS, RUBY GAP DUPLEX, CENTRAL AUSTRALIA

Sample Mineral K 40 Ar* 100 40Ar*/ K/Ar age 40Ar/39Ar Lithologic no. (wt%) (IO"9 40ArtOiai (Ma) total fusion unit and (wt%)

mol/g) (Ma)

age (Ma) sheet numberf

533 Phengite 8.98, 8.95 5.273 96.9 310.7 +3.2 311.510.4 Q6 797 Phengite 8.51, 8.36 5.060 97.0 316.5 ±3.7 318.1 ±0.6 Q5 733 Phengite 6.75, 6.88 4.110 95.2 317.8+5.0 Q3 764 Phengite 8.61, 8.45 5.246 98.4 323.7 ±3.8 321.7 ±0.5 Q3 731 Phengite 6.22, 6.20 4.514 97.1 376.9 ±3.9 Q1 Phengite

4.487 96.6 374.8 ±3.9 520 Phengite 8.32, 8.26 5.352 98.2 338.5 ±3.5 328.2 ±0.5 Q1 796 Phengite 8.97, 8.94 5.812 97.2 340.1 +3.5 G5 627 Phengite 6.89, 6.88 5.553 94.2 413.5 +4.3 G4 731 Muscovite 8.25, 8.30 35.92 99.8 1569 ± 1 6 Q1 627 Muscovite 9.07, 9.20 35.87 99.0 1467 + 25 1468 ±4 G4

36.05 97.9 1475 ± 2 5 796 Muscovite 9.21, 9.05 26.30 95.8 1178 ± 1 7 1237 ±2 G5

25.89 93.5 1166 ± 1 7 Note: Uncertainties in ages are quoted at l a level. Abundance of 40K/K,olai = 1.167 x 10~4 mol/mol.

Decay contants for 40K are: Xp = 4.962 x 10"10 yr"1 and A<e«') = ° - 5 8 1 x 10~10 y r 1 . •Radiogenic Ar. t Q = quartzite, G = granitic gneiss.

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grains. The breakdown of white micas during deformation is a complex process driven by both stored strain energy and chemical free energy (Etheridge and Hobbs, 1974). Grain size reduc-tion is accomplished in part by the mechanical disaggregation and dissolution of micas, al-though the role of dynamic recrystallization in the process of grain size reduction is unclear. Dissolution and neocrystallization of micas ap-pears to be more common in rocks deformed under low-grade conditions (e.g., Wilson and Bell, 1979; Kligfield et al„ 1986).

Micas generally deform by folding, fracturing, kinking, and disaggregating along cleavage. As pointed out by Etheridge and Hobbs (1974), dynamic recrystallization in phyllosilicates seems to be difficult to achieve because of the abundance of partial dislocations and the lack of unit dislocations. Thus, dynamic recrystalliza-tion by progressive subgrain rotation and grain boundary migration is not expected to be com-mon in micas. However, deformation of micas may result in segmentation of mica porphyro-clasts (Wilson and Bell, 1979), a process that does not destroy the original lattice and does not require pervasive dissolution, grain growth, or chemical change.

Significance of ^ A r / ^ A r ages of White Mica from the Ruby Gap Duplex

Discordance between ^ A r / ^ A r apparent ages of muscovite porphyroclasts and neocrystal-lized phengite from the Ruby Gap duplex is not consistent with formation of the phengite well above temperatures required for significant dif-fusive loss of argon and with subsequent cooling through the closure temperature. If temperature had significantly exceeded that necessary for significant loss of argon from white mica for an extended period of time, we would expect the muscovite porphyroclasts to be strongly reset. Because the young apparent age steps in the 4 0Ar/3 9Ar spectra of the muscovite porphyro-clasts can be explained at least partly by phengite contamination, we propose that the temperature did not exceed that at which signifi-cant argon diffusion occurs in muscovite during the formation of the Ruby Gap duplex.

Phengites in quartzite mylonites in the Ruby Gap duplex have clearly grown in response to deformation. Dissolution of detrital muscovite during deformation, and transport of the con-stituents in a fluid phase, with subsequent crys-tallization of new phengites, have resulted in complete loss of preexisting radiogenic argon. The neocrystallized phengites have apparently not incorporated any ambient argon into the lat-tice during crystallization. Phengite grains in the quartzite mylonites contain a strong preferred orientation and are relatively strain free, indicat-ing that they probably crystallized late in the strain history. Therefore, we interpret the age of these neocrystallized phengites as recording the

latest increments of deformation-induced recrys-tallization of the quartzite mylonites.

White mica porphyroclasts from the Ruby Gap duplex have retained most of their radio-genic argon despite having undergone intense Paleozoic deformation. Segmentation of white mica porphyroclasts into masses of finer grains is a nearly isochemical process that may involve only limited dissolution of the original lattice. Because the original lattice remains largely intact during segmentation, this process appears not to result in significant argon loss, which is consist-ent with our results. These findings appear to be in conflict with those of Behrman (1984), who suggested that segmentation is associated with nucleation and fluid-assisted isochemical growth of new white micas.

The isotopic ages obtained from the Ruby Gap duplex provide overwhelming evidence of major deformation-induced recrystallization during the Paleozoic Alice Springs orogeny in central Australia. Neocrystallization of the phen-gite during deformation, in the approximate temperature range 250-350 °C, leads us to pro-pose that the trend of phengite ages across the duplex records migration of tectonic activity toward the internal zones of the thrust system during the final stages of ductile deformation (i.e., reverse of piggyback sequence). This phenomenon took 15 m.y., from about 325 to 310 Ma, the age difference between the flat parts of the age spectra from sheet 1 and sheet 6 phengites. This constrains the late stages of struc-tural and microstructural development of the Ruby Gap duplex, and it demonstrates that thermochronological studies can give informa-tion bearing on the mechanical problem of for-mation of mid-crustal ductile duplexes (Teyssier et al., 1991).

REFERENCES CITED Behrman, J.H., 1984, A study of white mica micro-

structure and microchemistry in a low grade mylonite: Journal of Structural Geology, v. 6, p. 283-292.

Boyer, S.E., and Elliot, D., 1982, Thrust systems: American Association of Petroleum Geologists Bulletin, v. 66, p. 1196-1230.

Chopin, C., and Maluski, H„ 1980, ^Ar-^Ar dating of high pressure metamorphic micas from the Gran Paradiso area (western Alps): Evidence against the blocking temperature concept: Con-tributions to Mineralogy and Petrology, v. 74, p. 109-122.

Dodson, M.H., 1973, Closure temperature in cooling geochronological and petrological systems: Con-tributions to Mineralogy and Petrology, v. 40, p. 259-274.

Dodson, M.H., and McClelland-Brown, E., 1985, Iso-topic and palaeomagnetic evidence for rates of cooling, uplift and erosion, in Snelling, N.J., ed., The chronology of the geologic record: Geologi-cal Society of London Memoir 10, p. 315-325.

Dunlap, W.J., Teyssier, C„ and McDougall, I., 1990, Tectonic evolution of the Arltunga Nappe Com-plex, central Australia: An integrated structural and isotopic dating study [abs.]: Eos (Transac-

tions, American Geophysical Union), v. 71, p. 1596.

Etheridge, M.A., and Hobbs, B.E., 1974, Chemical and deformational controls on recrystallization of mica: Contributions to Mineralogy and Petrol-ogy, v. 43, p. 111-124.

Fischer, M.W., and Coward, M.P., 1982, Strain and folds within thrust sheets: An analysis of the Hei-lam sheet, NW Scotland: Tectonophysics, v. 88, p. 291-312.

Forman, D.J., 1971, The Arltunga Nappe Complex, MacDonnell Ranges, N.T., Australia: Geological Society of Australia Journal, v. 18, p. 173-182.

Kligfield, R., Johannes, H., and Dallmeyer, R.D., 1986, Dating of deformation phases using the K-Ar and 40Ar/39Ar technique: Results from the northern Apennines: Journal of Structural Geol-ogy, v. 8, p. 781-798.

Robbins, G.A., 1972, Radiogenic argon diffusion in muscovite under hydrothermal conditions [M.S. thesis]: Providence, Rhode Island, Brown Uni-versity, 189 p.

Scaillet, S., Feraud, G., Lagabrielle, Y., Ballevre, M., and Ruffet, G., 1990, ^ A r / ^ A r laser-probe dat-ing by step heating and spot fusion of phengites from the Dora Maira nappe of the western Alps, Italy: Geology, v. 18, p. 741-744.

Shaw, R.D., Stewart, A.J., Yar Khan, M., and Funk, J.L., 1971, Progress reports on detailed studies in the Arltunga Nappe Complex, Northern Terri-tory: Bureau of Mineral Resources, Australia, Record 1971/66.

Stewart, A.J., 1971, Potassium-argon dates from the Arltunga Nappe Complex, Northern Territory: Geological Society of Australia Journal, v. 17, p. 205-211.

Teyssier, C., 1985, A crustal thrust system in an intra-cratonic tectonic environment: Journal of Struc-tural Geology, v. 7, p. 689-700.

Teyssier, C., Dunlap, W.J., and Karato, S.-I., 1991, Propagation of ductile deformation in thrust sys-tem: Structural and radiometric evidence and tec-tonic implications: Terra Abstracts, Supplement 5 to Terra Nova, v. 3, p. 39.

Urai, J.L., Means, W.D., and Lister, G.S., 1986, Dy-namic recrystallization of minerals, in Heard, H.C., and Hobbs, B.E., eds., Minerals and rock deformation: Laboratory studies: American Geo-physical Union Monograph 36, p. 161-199.

Velde, B„ 1967, Si4+ of natural phengites: Contribu-tions to Mineralogy and Petrology, v. 14, p. 250-258.

Wijbrans, J.R., and McDougall, I., 1986, "°Ar/39Ar dating of white micas from an alpine high-pressure metamorphic belt on Naxos (Greece): The resetting of the argon isotopic system: Con-tributions to Mineralogy and Petrology, v. 93, p. 187-194.

Wilson, C.J.L., and Bell, I.A., 1979, Deformation of biotite and muscovite: Optical microstructure: Tectonophysics, v. 58, p. 179-200.

ACKNOWLEDGMENTS Supported by National Science Foundation Grant

EAR-8720755 (Teyssier) and by the Australian Na-tional University Research School of Earth Sciences. We are grateful for support for irradiations given by the Australian Institute of Nuclear Science and Engi-neering. We thank T. M. Harrison, D. L. Kirschner, A. J. Stewart, and R. D. Shaw for reviews of an early version of the manuscript, and R. Maier, J. Mya, D. Patterson, H. Kokkonen, and R. Rudowski.

Manuscript received April 2, 1991 Revised manuscript received August 21, 1991 Manuscript accepted August 27, 1991

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