Differential Stress , Strain Rate , and Temperatures of Mylonitization in the Ruby Mountains , Nevada : Implications for the Rate and Duration of Uplift

Kno~ledge of the m.agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low-angle detachment faults. We have analyzed the differential flow stress during the mylonitization of quartzose rocks in the Ruby Mountains, Nevada, using grain-size piezometers and kinetic laws for grain g!owth. Quartzites from mylonitic shear zones in Lamoille Canyon and Secret Creek gorge have grain SizeS of 91-151 ~m and 42-64 ~m, respectively. The peak temperature during mylonitization was 630°:i:50°C, and analysis of grain-growth kinetics indicates that mylonitization continued during cooling to temperatures S450°C. Quartz grain-size piezometers suggest that the mylonitization occurred under differential stresses (crl-cr3) of 38-64 MPa, or maximum shear stresses of 19-32 MPa. Extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10-10 and 10-13.-1. . ' arguments presented m the paper suggest that the likely range of strain rates is 10-11 to 10-12.-1. These strain rates are compatible with displacement rates of the order of 23 mm yr-l along a 1.5-km-thick simple shear zone. Such a shear zone dipping 15° would produce an uplift rate of 5.8 km m.y:1 and a horizontal extensioo rate of 22 km m.y:l. This uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1.5-km-thick simple shear zone dipping 15° in 2.6 m.y.


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middle crust [e.g., Davis et al., 1986].The ductilely deformed pynamic processes within ductilely deformed middle midcrustal rocks were lifted up along the footwalls of the continental crust played an important role in mid-Tertiary core detachment faults to Earth's surface.Although the geometrical complex extension in the North American Cordillera [e.g., evolution of brittle detachment fault systems and the Coney and Harms, 1984].Knowledge of the state of stress in kinematics of the associated ductile shear zones have been the middle crust during the extension is fundamental to studied by numerous geologists, little is known about the understanding the physics of those processes.First, the differential stress and strain rate during the evolution of these differential stress provides a direct constraint on any deeper shear zones.mechanical modelling of crustal deformation.It can also be The purpose of this paper is to use theoretically derived and used, in conjunction with the temperature history of the rocks experimentally calibrated microstructural piezometry of quartz and experimental flow laws, to infer the strain rate during the to infer the differential stress during the mylonitization of development of ductile shear zones, which are common features quartzose rocks along a shear zone in the Ruby Mountains core in Cordilleran core complexes [e.g., Crittenden et al., 1980;complex.The strain rates, and rates of extension and uplift Frost and Martin, 1982;Snoke and Lush, 1984;Davis et al., during the development of the shear zone are then calculated by 1986].
applying quartzite flow laws at the estimated stresses and The core complexes and associated detachment fault systems metamorphic temperatures.have been intensively studied in the past decade [e.g., .

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south side of Nevada highway 229 (Figure 1).The samples were all collected within 10 m of the low-angle fault contact Lamoille Canyon that separates a lower plate of mylonitic interlayered impure The western end of Lamoille Canyon contains exposures of quartzite, migmatitic schist and orthogneiss from the the mylonitic zone that forms a carapace above the higher-overlying carbonate-rich Horse Creek allochthon.grade core of the range.This mylonitic zone is at least 1.5-2 All the samples are quartz-rich mylonites with thin layers km thick and can be traced along the west flank of the range for containing muscovite or biotite (0-5%) or plagioclase (1about 100 km [Valasek et al., 1989].Sedimentary rocks within 10%).
Several samples  show the mylonitic zone in this area have been attenuated to 5% of "ribbon" textures, with large quartz grains, greatly elongated their thickness outside of the zone [Snoke and Howard, 1984].parallel to the compositional layering (C), separated by fme-We collected five samples along a 300-m-long north-south grained recrystallized grains (Figure 2c).The linear intercept transect at approximately 115°28' and 40°41.5',about 300 m measurements (Table 1) do not include the large relict grains.east of the Lamoille Canyon road (Figure 1).The transect In these samples the amount of recrystallization varies from includes probable Mesozoic granite gneiss, Ordovician--20% in sample Ru-17, with relict grain dimensions parallel to Cambrian calc-silicate rocks of Verdi Peak, the Cambrian and the lineation of 0.3 x 6.0 mm, to 80% in sample Ru-12, with Late Proterozoic Prospect Mountain Quartzite, and the garnet-relict grain dimensions of 0.2 x 15.0 mm.The elongation of two-mica granite orthogneiss of Thorpe Creek.
All the the small recrystallized grains defines a weak foliation (S) that samples we collected are of Prospect Mountain Quartzite or (measured in sections parallel to the lineation and normal to quartzose layers in the orthogneiss of Thorpe Creek. the foliation) is parallel to C in samples Ru-12, Ru-16, Ru-18, The samples are all strongly foliated quartzose mylonites and Ru-19, and inclined to C at angles of 15°, 20°, and 27° in with up to 5% muscovite and/or biotite and 5% plagioclase, s~ples Ru-10, Ru-11, and Ru-15, respectively.Plagioclase is generally segregated in thin layers defining a compositional present as anhedral or augen-shaped, cracked porphyroclasts.

foliation. They have been termed S-C mylonites by Lister and
The grain-boundary configurations and internal structures of Snoke [1984], the S (schistosite) being a planar structure quartz in the rocks from Secret Creek gorge differ somewhat defined by the shape anisotropy of finely recrystallized quartz from those from Lamoille Canyon.The "ribbon" grains show, and C (cisaillement) the compositional layering defmed by the in addition to some blocky subgrain structure, continuous mica and plagioclase crystals.The mica crystals are anhedral undulatory extinction or bending.The recrystallized grains, and lozenge-or fish-shaped.Plagioclase porphyroclasts are which are less than half the size of those in the Lamoille anhedral and augen-shaped with internal fractures and Canyon mylonites, also show slight, continuous undulatory deformation bands.The S foliation is inclined at -20° to the extinction (less than 10° and commonly less than 5° of compositional interlayers (C).The quartz grains have a strong bending in a single grain), without optically visible subgrains lattice preferred orientation, as indicated by the uniformity of in most of the samples (e.g., Figure 2d).Undulatory extinction colors produced by a gypsum plate; examples are illustrated by is essentially absent in samples Ru-12 and Ru-18. This Snoke [1980, Figure 10]  recrystallization, or more prolonged plastic deformation after Samples Ru-4, Ru-5, and Ru-6 show bimodal grain-size recrystallization, than in the Lamoille Canyon mylonites.The distribution with a few relatively large, flattened relict grains, grain boundaries are commonly irregular or sutured, indicating surrounded by smaller recrystallized grains of uniform size grain boundary migration during or after deformation.
In dating from the mylonitic deformation.The relict grains are general, the textures of the Secret Creek gorge mylonites generally elongate parallel to the foliation (S), with minumum suggest deformation and recrystallization at lower dimensions >0.5 x 2.0 mm, indicating that the premylonitic temperatures, with less postmylonitic grain growth than in the proto lith was coarse grained (»1.0 mm).They show Lamoille Canyon mylonites.
The large size of the quartz extensive undulatory extinction of the "blocky" type, defined ribbons and of the mica and plagioclase porphyroclasts by unbent subgrains separated by relatively high-angle indicates a coarse-grained (~2 mm) premylonitic protolith for subgrain boundaries.This is a well-recovered substructure.The most of these rocks.recrystallized grains, by contrast, show little or no undulatory ., .. extinction.
Their grain boundaries range from somewhat Gram Size DetermlnatlOn serrated (specimen Ru-7, Figure 2a), indicating grain boundary The recrystallized grain sizes were measured by the method of migration during or after the dynamic recrystallization that Ord and Christie [1984].Two I-inch round polished ~afers accompanied mylonitization, to straight (specimen Ru-8, were cut from each sample.Both wafers were c:ut ~ndlcular Figure 2b), suggesting postmylonitic grain growth.Samples to the foliation; one was cut par~lel t? the lmeatlon and the Ru-7 and Ru-8 have a unimodal grain size distribution, but the other was cut orthogonal to the lmeatlon.
Each was etchp resence of subgrain structure in a few grains suggests that they with 40% ammonium bifluoride for ~5 min to .revealgram may be relict grains.
boundaries, but not low angle sub-gram boundaries [Wegner  for a single sample is the geometric mean of 20 inverse mean .linear intercepts (each of -25-50 grains) and is not the is geometric mean that would be obtained by measuring grains ĩndividually.
Grain-shape ellipsoids range from 1.0: 1.0:0.9 to We used the recrystallized-grain-size piezometers of Twiss +5 +3 +12 +32 [1977,1980], Mercier et al. [1977], and Koch [1983]  e experlfllen a ca 1 ra ion error an e s an ar deviations of the grain-size measurements.Moreover, we used the same grain-size measurement technique that Koch used, so that our grain sizes are directly comparable to Koch:s.
7 and 9 MPa.Differential stresses for the rocks from Secret Differential stresses for.the rocks from Lamo1l1e Canyon Creek gorge range from 31 to 64 MPa, with eight of the 10 range from 7 to 17 MPa, With four of the five samples between samples between 35 and 49 MPa.Note that the uncertainties of these values are in some cases as large as the values themselves.Because the grain size may have increased during  1977].Invariably, grains are larger after armealing than they Twiss [1977,1980] 1.45xl04 -1.47 were during deformation, and the larger, annealed grain size .
3 leads to an underestimate of the actual flow stress (see Table 3).growth during armealing can be evaluated.minimum grain size that could have resulted from Estimates of the peak temperature and pressure during mylonitizatio~ at 540°C, corresponding to a differential stress mylonitization, determined from garnet-biotite-muscovite-of 2: ~Pa (Flgur~ 4): .° °plagioclase thermo barometry, are 630°:t50°C and 400:1:100 Similar reasonmg mdlcates that T a=450 C and T max=500 C MPa [Hurlow, 1988; H. Hurlow, personal communication, for the mylonitic rocks in Secret Creek gorge with a final grain 1989].Consideration of the grain-growth kinetics of quartz size o~ 57 ~m.Gr~~ of 5:-~~ diameter can form during aggregates, however, indicates that mylonitization must have annealmg from any mltlal graIn Size smaller than 57 ~m.At continued to lower temperatures.The average grain sizes of the S450°C, 57-~m grains do not grow, thus T a=450°C for this Lamoille Canyon and Secret Creek gorge rocks are 146 and 57 grain size and cooling rate.During cooling from 490°C, grains ~m, respectively.From the kinetic laws for the grain growth that were initially 43 ~ grow to a final grain size of 57 ~m. of quartz (fable 4) [Tullis and Yund, 1982; Pierce and Christie At temperatures ?500°C, grains rapidly grow to more than 57 growth history for grains with a final diameter of 146 and 57 limit (T max) on the temperature at the end of mylonitization of ~m developed during fiXed cooling rates (Figure 4). Figure 4 the rocks in Secret Creek gorge of 490°C, based on grainshows several grain growth paths for cooling from different growth kinetics.Thus 42 ~m is the minimum grain size that temperatures at a linear rate of 54°C m.y.-1 (the minimum could have resulted from mylonitization at 500°C, corresponding to a differential stress of 64 MPa (Figure 4).
Let  Quartzite, name of the rock on which the measurements were made; Kronenberg and Tullis [1984] used both Heavitree Quartzite and Arkansas novaculite.%H20, amount of water added to the samples; talc indicates experiments in which the samples were surrounded by dehydrating talc.No., number of samples on which measurements were made.
* Includes data from eight experiments by Heard and Carter [1968].t Preexponential constant from Kirby and Kronenberg [1987] same mechanisms that operated during the experiments, then observed grain sizes and the grain growth calculations above.the constitutive relations can be used to predict one of the At temperatures between T max and T g, the predicted strain rates variables, temperature, stress, or strain rate, if the other two for the rocks from Lamoille Canyon are in the range 10-9 variables are known [Poirier, 1985].Flow laws for steady state to 10-14 s-I, and those for rocks from Secret Creek gorge dislocation creep of quartzite are listed in Table 5.We did not are 10-9 to 10-12 s-1 at these temperatures.At a lower include flow laws for vacuum-dried samples, because the temperature, 400°C, strain rates are one order of magnitude presence of biotite and muscovite indicates that the natural slower.The strain rates derived from the experiments of Koch samples were probably not deformed under anhydrous et al. [1989] are the slowest and yield the most conservative conditions.We have also excluded rheological data from extrapolations.
Even with the maximum uncertainty experiments on novaculite and flint.None of the sets of (including grain-size measurement errors, piezometerexperiments from which the flow laws were derived were ideal.calibration errors, and flow-law calibration errors), strain rates All were done in solid-medium apparatus, which can not of 10-13 s-1 or faster are predicted for the mylonitic rocks from measure stress as accurately as can gas apparatus.Kronenberg Secret Creek gorge.and Tullis ' [1984] and Jaoul et al. 's [1984] experimental samples were encapsulated in platinum, which affects the stress Implications for the Rate and Duration of Uplift measurements during the experiments.
Further, their If the thickness of the mylonitic shear zone that produced the rheological data come from creep experiments on five or fewer mylonitic rocks is known, from the strain rate we can calculate samples.Koch et al.'s [1989] experiments were done with the displacement rate across the shear zone.The mylonitic copper or copper and talc confining media, which are tstronger zone within the Ruby Mountains has a total thickness of 1.5-2 than the salt confining medium used in some of the other km [Valasek et al., 1989].However, field relationships in experiments.Although experiments have shown that the Secret Creek gorge indicate that as the mylonitic rocks cooled, rheological behavior of quartz is affected by pressure, strain localization occurred, and the fault zone became impurities such as Na [Jaoul, 1984], water content [Jaoul et al., progressively narrower [Snoke and Lush, 1984].The rocks 1984; Kronenberg and Tullis, 1984], and the a/13 transition that we sampled from Secret Creek gorge represent some [Linker and Kirby, 1981;Ross et al., 1983], none of these unknown, presumably intermediate step in the transition from effects can yet be extrapolated quantitatively to natural a thick, homogeneously deforming zone to a thinner shear conditions.

TABLE 1 .
Number of Grains Counted, Geometric Mean Grain Aspect Ratios, and Geometric Mean Grain Size Grain size estimates were made from mean linear ~ ~ too = u intercepts [Smith and Guttman, 1953] of 500-1000 grains per ~

TABLE 4 .
Experimentally Detemlined Parameters for Kinetics of Grain cooling rate given by 40 Ar/39 Ar and fission track data for the Growth in Flint and Novaculite mylonitic rocks in Lamoille Canyon, as explained earlier).

TABLE 5 .
Experimentally Detennined Parameters for Power Law Creep Constitutive Equations for Quartzites

TABLE 7 .
Shear Zone Parallel Displacement Rates .