Journal of Structural , Vol. 16. No. 4, pp. 555 to 57t1, 1994 Pergamon Elsevier Science Ltd Printed in Great Britain 0191-8141/94 $6.00+0.0(I

0191-8141(93)E0013-B

Constrictionai strain in a non-coaxial zone: implications for and development, central Mojave metamorphic core complex, California

JOHN M. FLETCHER and JOHN M. BARTLEY

Department of Geology and Geophysics, 717 William Browning Building, University of Utah, Salt Lake City, UT 84112-1183, U.S.A.

(Received 3 February 1993; accepted in revised form 16 November 1993)

Abstract--Finite strain and fold analyses of footwall in the central Mojave metamorphic core complex (CMMCC) reveal two distinct deformation paths that generated non-coaxial constrictional strain. The first path, recorded in the Waterman Hills, accomplishes constrictional strain through the formation of L-tectonites at the grain scale. The second path, recorded in the Mitchel Range, involves a combination of plane strain at the grain scale and Y-axis shortening through syn-mylonitic folding. The ductile in the Waterman Hills is approximately 500 m thick and is developed entirely within a Miocene syn-kinematic granodiorite. Strain magnitude decreases structurally downward, The mylonites contain abundant NE-directcd non-coaxial microstructures yet are L-tectonites. Rf/ep analysis of quartz-ribbon grain shapes indicate prolate strains (K=6) and bulk chemical data suggest that mylonitization was isochemical or involved volume loss. Therefore, ductile shearing in the Waterman Hills involved true constrictional strain at the grain scale. Immediately to the southeast in the Mitchel Range, the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre-Tertiary basement. The mylonitic fabric comprises a well-developed and NE-trending stretching [ineation. Rf/(a strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite. In contrast to the Waterman Hills, finite strains in these L-S-tectonites approximate plane strain (K = 1.1). However, the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching . The coaxial folds range from open to isoclinal. We interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished Y-axis shortening in the shear zone. Macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone, suggesting that Y-axis shortening continued after the zone had reached the brittle regime. Corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in Cordilleran metamorphic core complexes. Therefore, constrictional strain may be common in core complexes. Strain compatibility with upper plate rocks in core complexes may be maintained by Y-axis shortening along conjugate strike slip faults. A nearly uniaxial field (a3 < 02 ~ ol) is consistent with most of the structures formed in Basin and Range extension and could generate the observed constrictional strain.

INTRODUCTION record a more complete deformational expression of the large-scale tectonic regime. In other words, we suggest DEFORMATION paths in shear zones are commonly inter- that a single ductile shear zone may accomplish strains preted in terms of the ideal shear zone model (Ramsay & that could only be produced by a network of brittle faults Graham 1970, Ramsay 1980). In the model, ductile with some combination of dip-slip and strike-slip dis- shear zones are the kinematic equivalent of brittle fault placements. zones: the shear zone is bounded by rigid blocks that are The basin-and- geometry of the brittle-ductile displaced relative to each other. As a result, defor- faults in Cordilleran metamorphic core complexes is the mation is dominated by simple shear. This geometry result of the superposition of two upright fold sets requires plane strain, i.e. no true strain along the inter- (Spencer 1982, Yin 1991, Yin & Dunn 1992). One set mediate principal strain axis (Y-axis) of the shear zone. comprises long-wavelength folds with axes that trend Apparent departures from plane strain are accom- perpendicular to the movement direction. Most workers plished through dilatation of the shear zone perpen- agree that this folding of the fault zone occurs in re- dicular to its walls (Ramsay & Wood 1973). Apparent sponse to tectonic denudation and isostatic rebound of flattening strains are produced by volume loss and the footwall (Spencer 1984, Buck 1988, Hamilton 1988, apparent constriction by volume gain. Wernicke & Axen 1988, Bartley et al. 1990, Reynolds & We report field and laboratory studies of mylonites in Lister 1990). These structures clearly overprint mylon- the extensional shear zone of the central Mojave meta- itization and active displacement across the fault zone morphic core complex (CMMCC). The results indicate and are not considered further in this paper. The second true Y-axis shortening that cannot be interpreted in set consists of shorter-wavelength folds oriented parallel terms of the ideal shear zone model. We argue that, to the movement direction. The genesis of these folds is besides accomplishing relative displacement of the controversial. Some workers interpret the folds to form bounding blocks, crustal-scale ductile shear zones may as primary corrugations in the fault zone (John 1987, 555 556 J.M. FLETCHERand J. M. BARTLEY

Llthologles: E:tce2:ion-related Miocene sedimentary and volcanic

~ Variably mylonitic mid-Tertiary granodiorite ~lMylonitic pre-Tertiary basement \ .onmy,on.,ooro-Tertiar, basemont

--C M M C C ¸

'~'~"< #'~.;'q.~Figure 2 F Barstow .

0, 5, io, ,5, zo Southern California Km.

Fig. 1. Schematic geologic map of the central Mojave metamorphic core complex (CMMCC). Area ornamented with wavy lines is alluvium-covercd but inferred to be underlain by Miocene . B--Buttes, CM--Calico Mountains, FP-- Frcmont Peak, GH--Gravel Hills, HH--Hinkley Hills, IM--Iron Mountain, LM--Lane Mountain, MH--Mud Hills, MR--Mitchel Range, WH--Waterman Hills, SAF---San Andreas Fault, GF--Garlock Fault.

Spencer & Reynolds 1991). Others argue that the mylonitization involved 23 + 0.9 Ma dacite dikes movement-parallel folds formed by secondary folding of (Walker et al. 1990) and the 18-30 Ma Barstow Forma- an originally sub-planar fault zone (Spencer 1982, Yin tion (Burke et al. 1982, MacFadden et al. 1990) was 1991, Yin & Dunn 1992). Our data support the latter deposited post-kinematically in the basin (Glazner et theory and indicate that folding occurred during active al. 1989). This study focuses on exposures of the ductile displacement across the brittle-ductile fault zone in the shear zone at the southeastern end of the core complex CMMCC. We interpret this folding and extensional in the Waterman Hills and Mitchel Range (Fig. 2). faulting to reflect a large-scale constrictional strain The system is laterally continuous in regime in which horizontal extension occurred along the this area and composed of four main structural com- azimuth of fault slip and shortening occurred along ponents. Brittle faults, including the detachment and vertical and horizontal axes perpendicular to the azi- upper-plate faults, form the structurally highest ele- muth of fault slip. ments. The detachment juxtaposes mid-Tertiary vol- This study presents finite strain and fabric analyses canic and sedimentary rocks in the hangingwall with from two along-strike segments of the ductile shear zone mylonitized crystalline basement in the footwall. A zone in the CMMCC that record two different constrictional of penetrative cataclasis extends 50-100 m below the strain paths. We reevaluate existing models for syn- detachment-fault surface. Ultramylonites that record mylonitic folding and outline deformational processes extreme grain-size reduction and significant chemical that occur in a shear zone undergoing constrictional alteration (Glazner & Bartley 1991) occur in a 5-10 m strain. Finally, we suggest that constrictional strain may thick zone immediately below the detachment fault. be a common product of extensional tectonism. Finally, greenschist-facies mylonites occur in the ductile shear zone that reaches a thickness in excess of 1000 m. As observed in all Cordilleran metamorphic core com- CENTRAL MOJAVE METAMORPHIC CORE plexes, brittle deformational fabrics overprint ductile COMPLEX fabrics. The brittle-ductile fault system does not change from The CMMCC is characterized by a NE-directed de- the Waterman Hills to the Mitchel Range except for the tachment fault system that can be traced 40 km along character of mylonitization in the ductile shear zone. In strike (Fig. 1). Normal-sense displacement across the the Mitchel Range myionitization primarily affected fault zone separated regional pre-Tertiary lithologic heterogeneous pre-Tertiary basement that consists of markers in the hanging wall and footwall by approxi- interleaved meta-sedimentary and meta-igneous rocks. mately 40-60 km (Glazer et al. 1989, Martin & Walker Meta-sedimentary units include quartzite, pelite and 1991). This extension occurred between 23 and 18 Ma: marble which are tentatively correlated with late Constrictional strain in a non-coaxial shear zone 557

( , , , --7 / \'~:i ', ': , :' , ,' ',,~Waterman Hills

~ ~)-:' , ...... , ,%

@ "///////U~ / //~'

Range

~ Miocene and vol ~ Waterm (isotro I f>:i ~ Waterm

Interprc show,nc Z~ Penelra j Detachm~rl[ TSUI[ \ Synform

Barstow

Fig. 2. Schematic geologic map of the Waterman Hills and Mitchel Range. Miocene granodiorite in the Waterman Hills contains an L-tcctonitc fabric. Prc-Tertiary rocks in the Mitchel Range arc L-S-tectonites folded about axes subparallel to the stretching lineation.

Proterozoic-Paleozoic Cordilleran miogeoclinal strata the interpretation that the granodiorite was emplaced (Stewart & Poole 1975, Kiser 1981). Meta-igneous rocks during mylonitization. The remainder of this paper range in composition from gabbro to leucogranite with outlines and compares the structural evolution of these hornblende metagabbro forming the dominant lithology two shear zone segments. in the range. Although these rocks are almost com- pletely overprinted by Tertiary greenschist-facies mylonitization, porphyroclasts in pelitic rocks preserve SYN-MYLONITIC DEFORMATIONAL FABRICS a relict upper amphibolite-facies assemblage of garnet + biotite + muscovite + quartz _+ kyanite _+ sillimanite _+ The monolithologic ductile shear zone in the Water- staurolite from an earlier, probably Mesozoic, thermo- man Hills is approximately 500 m thick and displays a tectonic event. The issue of distinguishing strains associ- strong strain gradient with structural level. The grano- ated with Mesozoic deformation from mid-Tertiary diorite is essentially isotropic and undeformed in the mylonitization is discussed later. structurally lowest exposures and develops a progres- In contrast to the lithologically heterogeneous mylon- sively more intense mylonitic fabric upward toward the ires in the Mitchel Range, the ductile shear zone in the brittle detachment (Bartley et al. 1990, Glazner & Bart- Waterman Hills only affects a medium- to coarse- ley 1991). Lithologic markers such as aplite dikes con- grained granodiorite pluton, dated by the U/Pb zircon tain the mylonitic fabric but are not rotated into method at about 22-23 Ma (Glazner et al. 1992). Pri- parallelism with it. mary minerals in the granodiorite include euhedral The ductile shear zone in the Mitchel Range is much cyclically-zoned plagioclase, quartz, biotite and minor thicker: its lower boundary is not exposed in about 1000 hornblende with poikilitic orthoclase overgrowing all m of structural relief. However, the shear zone contains other phases. Late myrmekite partially replaces ortho- macroscopic blocks of relatively unmylonitized rock, clase. Most of the important differences in character indicating that strain is heterogeneous within the zone. between the two shear zone segments are consistent with Most of the shear zone is characterized by mylonitic 558 J.M. FLETCHERand J. M. BARTLEY

(c)

( - +

Waterman Hills stretching Mitchel Range stretching Mitchel Range mesoscopic fold lineations (n=140) lineations (n=343) hinges(n=65)

Mitchel Range mylonitic Mitchel Range mesoscopic foliations (n=678) fold-hinge surfaces (n=52) Fig. 3. Myloniticfabrics from the Waterman Hills and Mitchel Range. Data contoured by Kamb method on equal-area sterconcts. (a) Contour interval = 2.0 sigma, (b) contour interval = 5.0 sigma, (c) contour interval = 2.2 sigma, (d) pole to great-circle girdle is 14-238. Contour interval = 3.5 sigma. (e) pole to great-circle girdle is 18-247. Contour interval - 1.2 sigma. gneisslc banding. Lithologic markers, which include and include S-C fabrics, C' shear bands, sigma- and early Miocene dacite dikes, are generally completely delta-type augen, mica fish arrays and domino-block rotated into parallelism with the mylonitic fabric. Only rotations in fractured hornblende, feldspar and garnet sparse, as-yet undated felsite dikes are cross-cut by the porphyroclasts (Bartley etal. 1990, Dokka 1989). Mylo- fabric but not fully transposed. Rotation and trans- nitization in both segments involved transitional brittle- position of compositional layering occurred largely plastic deformation mechanisms (Bartley et al. 1990): through synmylonitic folding which is discussed in a later quartz is dynamically recrystallized and shows evidence section. for both dislocation creep and dislocation glide; feld- The mesoscopic character of the mylonitic fabric spars are cataclastically deformed and syn-kinematicaily differs in the two shear zone segments, The mylonites in replaced by cryptocrystalline aggregates of quartz, white the Waterman Hills segment are nearly pure L- mica, and plagioclase; and micas are kinked and sheared tectonites that contain no measurable foliation (Bartley into arrays of fish with separations larger than the et al, 1990, Glazner & Bartley 1991). The stretching dimensions of the standard thin section. 4°Ar/39Ar re- lineation plunges northeast and southwest (Fig. 3a). The lease spectra constrain peak temperatures of mylonitiza- Mitchel Range segment is predominantly composed of tion to 300-400°C; phengitic white mica yields a L-S-tectonites that are defined by a prominent stretch- disturbed spectrum of about 52 Ma while biotite is ing lineation and an equally well developed foliation totally reset to 20-22 Ma (J. M. Bartley & W. J. Taylor (Dokka 1989). However, strongly rodded L-tectonites unpublished data). are exposed locally. The stretching lineation in the In summary, similarities in the stretching lineation Mitchel Range is subparallel to that in the Waterman orientation, NE-directed shear-sense and deformation Hills (Fig. 3b). mechanisms of mylonitization suggest that the laterally Despite the significant differences in the character of contiguous shear zone segments in the Waterman Hills the mylonitic fabric, both shear zone segments record a and Mitchel Range developed coevally in the same significant component of NE-directed non-coaxial deformational regime. The ductile shear zone in the strain. Rotational kinematic indicators are abundant Waterman Hills displays a continuous upward increase Constrictional strain in a non-coaxial shear zone 559

Symbols ! ' .: 7 ,~i i A detachment faull

/ mylonitiClineationstretching ; :~.~

.,~ mylonitic foliation and ~~"::~:~ stretching lineation /ithologies o 0.5 1.0 I I I Miocene volcanic and sedimentary rocks km, J

granodiorite (22 Ma.) ultramylonitic ~..: ' 50 LS- and L-tectonites

granodiorite (22 Ma.) protomylonitic L-tectonites ~ 2%-o~oo° ~%-o° ~ %o° ° °oo° ° °oo° ~°oo° ~ %g ° °o$ ° ~ granodiorite (22 Ma.) nonmylonitic

Fig. 4. Geologic map of the Waterman Hills showing locations of samples collected for the Rf/q) strain analysis (Wl-Wl3). Samples collected near the detachment yield the highest strain magnitudes. in fabric intensity but, aside from a thin zone of ultra- Mitchel Range segment yielded mean susceptibilities mylonite beneath the detachment fault, strain magni- between 10 3-10 -5 cgs units and both prolate and oblate tudes were not high enough to transpose mesoscopic anisotropy ellipsoids. In general, increases in mean compositional markers. In the Mitchel Range segment, susceptibility are positively correlated with increases in variations in fabric intensity occur without any simple the degree of anisotropy, suggesting that both the shape relationship to structural position, and compositional and magnitude of the susceptibility anisotropy reflect markers are typically transposed into parallelism with variations in magnetic mineralogy and not variations the mylonitic fabric. Qualitatively, the L-S-tectonites in in finite strain (Borradaile 1987, Borradaile & Sarvas the Mitchel Range appear to record higher strain magni- 1990). tudes and a larger flattening component than L- Strain determinations using porphyroclast reconstruc- tectonites in the Waterman Hills. tion might have been attempted on the Mitchel Range mylonites which contain abundant microcracked and boudinaged feldspar porphyroclasts with quartz fibers FINITE STRAIN ANALYSIS defining separations. However, the technique was not pursued because of observed microstructurcs that could Methodology cause erroneous strain estimates. Quartz fibers between widely separated fragments were commonly completely Several techniques were considered for measuring attenuated and thus made a large fraction of the total finite strain in the metaigneous mylonites, including extensional strain difficult to recognize. Strong deflec- magnetic susceptibility anisotropy (AMS), porphyro- tions of the external foliation around porphyroclasts clast reconstruction, Rf/cp analysis of distorted primary- suggest that they resisted shortening strains. Finally, grain shapes and normalized-Fry analysis of primary- strain accommodated in the cryptocrystalline alteration grain distributions. We concluded that Rf/qo analysis of products of feldspar could not be measured in such grain shapes yielded the most reliable results. Strain reconstructions. determinations using AMS were problematic. Mean In the Waterman Hills segment, the study was re- susceptibilities in the granodiorite were too low (of the stricted to the early Miocene granodiorite. Samples order of 10 -s cgs units) for precise AMS measurements were obtained at different structural levels to document with the available instrument. Mylonitic gabbro in the the vertical strain gradient (Fig. 4). In order to compare 560 J. M. FLETCHER and J. M. BART1.EY

::::::::::: ° Ooo ° o o° ooo OLOo OLOo 0OoO :i:i:i:i::" o o ~ ° o °o° o o o ~ o o ~ o o o °/ ~OoO Oooo OoooOOo~o~. ~ ,~o°O"'~ oo~

I~ °o o Io°? ~Oo o(

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a • . :i i iiiiiiii~i~ililili:::ii~! • .x, ~f i:i:: :: :::::::::::::::::::::::::::::::: • "\:: " :::::::::::::;i::i:i:i~:~::'.~ • "..======:::::::::::::::::::::::::::::::::_ s ym bo ,~ ""~ ~:~iii!:::~ detachment fault . "~ ~iiiiii~

.,~-Jl~" axial surface of open "" ,ti::iiS antifor m/synfor m ~i .~.. ~ axial surface of tight overturned antiform/synform

mylonitic foliation and stretching lineation Lithologies

Terliary volcanic and sedimentary rocks 0 015 liO myfonitic pre-Tertiary crystalline rocks km. metasedimentary rocks: petite, quartzite, marble Td~ Tertiary dacite porphyry (23.1 +0.1 Ma), aohanitic felsite 'g~ leucogranite: garnet-bearing two-mica granite ,~ ( >LoT • .~\ ms \ /oO/ nonmylonitic pre-Tertiary basement b

A A'

~,..~.P ....."'....~ii~::~::~ "::i-~:i::".~."~ .."'"".:i... . to.O ..~,~ .... ~"~-~-7-~~:~ .8 D ,~...... ;:'"~:.,... " ..:i~::::@t,~.::;:;,;...... :.:.:::: .!~....._.:.:.:.:.:.:-:;:::;:+:+:.:+:~..:.:.:.:~.,:+:..:::.:.::.....,: " ,'-" .~

0 0.5 I.O L, | I km.

Fig. 5. (a) Geologic map of the Mitchel Range showing locations of leucogranitc samples collected from the R~/~ strain analysis. Cross-section A-A' drawn perpendicular to movement. Anliformal and synformal undulations in the brittle detachment surface correspond to type 1 folds in the ductile shear zone.

finite strains between the two areas, a two-mica garnet- Hills (Figs. 6a & c). However, quartz markers in the bearing granite with a similar quartz:feldspar ratio to peraluminous granite from the Mitchel Range could not the granodiorite was examined in the lithologically be used because higher strain magnitudes made it diffi- heterogeneous Mitchel Range, The pre-Tertiary granite cult to distinguish pressure shadows from primary quartz occurs as a one meter thick unit that can be traced for ribbons. Well-developed quartz pressure shadows several kilometers along strike. Therefore, samples suggest that silica was either added to the bulk rock or from the Mitchel Range were collected from essentially transferred out of primary quartz grain domains. Thus, one structural level in the same zone (Fig. 5). quartz grains in the granite were affected by volumetric Deformed primary quartz grains were the most dis- strains that would be difficult to assess. Instead, garnet tinctive markers in the granodiorite from the Waterman porphyroclasts that were penetratively cataclasized and Constrictional strain in a non-coaxial shear zone 561 strongly altered to fine-grained aggregates of chlorite, in the whole-rock geochemistry of mylonitized and biotite and opaques were abundant and made excellent unmylonitized granodiorite; only the ultramylonites markers (Figs. 6b & d). We infer that these fine-grained adjacent to the brittle detachment underwent large vol- micaceous aggregates deformed primarily as passive ume changes. Additionally, quartz-filled tensile cracks markers because the external foliation in the matrix is are rare suggesting that closed-system only weakly deflected around them. However, it is likely was not an important and that that alteration and cataclasis occurred at some point volume was also conserved in grain domains. Therefore, during deformation and that the garnet porphyroclasts the L-tectonites record true constrictional strain which may not record strain from the earliest stages of defor- requires shortening along both the Y and the Z principal mation. This inaccuracy should only affect the absolute strain axes. magnitude of the strain determinations if the axial ratios Although the mylonitized granite in the Mitchel of the three-dimensional strain ellipsoid remained con- Range crops out only at one structural level in the shear stant during progressive deformation. A possible factor zone, measured strain in the L-S-tcctonites varies suf- that could affect the character of the measured finite ficiently to define a radial trend on a Flinn diagram near strain is boudinage of the markers due to stretching plane strain (Fig. 7). Strain magnitudes in the Mitchel parallel to the X axis. This would cause the maximum Range segment are high and marker ellipticities in the elongation to be underestimated and shift the measured XZ principal plane are commonly greater than 50:1. finite strain ratios toward the flattening field. Bulk-rock volumetric strains in the mylonitized per- Sample preparation for Rf/c/) and normalized-Fry ana- aluminous granite are difficult to assess because its lyses involved cutting large (20-30 cm) hand samples undeformed protolith has not been identified. How- along three mutually perpendicular faces subparallel to ever, whole-rock geochemistry of mylonitized and the XY, YZ and XZ principal planes and tracing the unmylonitized gabbro in the Michel Range indicates strain markers on transparent overlays on the wetted either isochemical behavior or w)lumc loss, similar to slab surfaces. The tracings were then digitized. A least- the Waterman Hills granodiorite (A. F. Glazner squares best-fit ellipse was calculated for each marker unpublished data). Volumetric strain also could have outline as well as its relative position and orientation. occurred in garnet grain domains as a result of the Approximately 50-120 grains were measured on each breakdown of garnet to chlorite and biotite. We are principal plane. From these data scatter plots for the currently in the process of quantifying this component normalized Fry (Ersley 1988) and Rf/(p (Dunnet 1969) of strain through characterization of phase chemistry methods were generated. and metamorphic reactions but presently it remains Tectonic strains were determined from the chi- undetermined. Although volumetric strains are not squared minima of the Rdp analyses (Peach & Lisle well constrained in the Mitchel Range mylonites, it is 1979). The chi-squared tests typically yielded initial- unlikely that they are sufficient to cause the difference ellipticity estimates of 1.72. Symmetrical Rt./(p plots in strain paths between the Waterman Hills and indicate no recognizable pre-existing preferred orien- Mitchel Range. In other words, an anisotropic Z axis tation of strain markers in either the Miocene granodior- w)lume loss of nearly 85% is required to produce an ite or the pre-Tertiary granite. Finite strains determined apparent plane-strain path from a true constrictional- from the normalized Fry technique were primarily used strain path with a K value of 6. We present micro- to check the Rf/p estimates. The precision of these structural evidence below which suggests that the results was poor but they crudely agree with the Rf/c/) Mitchel Range mylonites record no Y axis strain and determinations. This agreement suggests that the finite that the Rf/¢) strain estimates accurately reflect the true strains recorded by the markers used in the Rf/dp grain- tectonic strains. shape analysis are roughly equal to those of the matrix and that the strain markers deformed passively. Lastly, Microstructural indicators of finite strain the Rt/cb strain estimates from three subperpendicular planes were used to calculate a finite-strain ellipsoid according to the modified least-squares technique of Microstructures in thin sections cut subparallel to the ()wens (1984). These results are summarized in Table 1. principal planes of finite strain were compared to the strain paths predicted from Rf/~ determinations. Results Specifically, we identified how extension and contrac- tion were accomplished at the grain scale along the X The L-tectonites in the Waterman Hills yield strongly and Z axes, respectively, and searched for comparable prolate finite strain ellipsoids that plot well within the microstructures along the Y axis. The sign of elongation field of apparent constriction (Fig. 7). The data define a along the Y axis qualitatively indicates whether the true radial trend with a K value of about 6. Finite-strain finite strain is constrictional, flattening or plane strain. magnitude is systematically related to structural level in Microstructures that indicate contraction include con- the shear zone: larger strain magnitudes are recorded at centration of mica and deflection of external foliation progressively higher levels. Glazner & Bartley (1991) around competent porphyroclasts, preferred orien- interpret the volumetric strain in these mylonites to be tations of mica and elongate quartz subgrains in the negligible because they found no significant differences mylonitic matrix, and domino-style shear displacements 562 J.M. FLETCHERand J. M. BARTLEY across microcracks. Dilation across microcracks, micro- estimates of plane strain in the Mitchel Range mylon- boudinage, pressure shadows and secondary quartz ites. fibers indicate directions of extension. Qualitative differences in strain magnitude are defined by the rela- tive degree of grain-size reduction, deformation- MESOSCOPIC AND MACROSCOPIC enhanced chemical alteration and microstructural COMPONENTS OF FINITE STRAIN: development. SYN-MYLONITIC FOLDS The sheared granodiorite in the Waterman Hills seg- ment has a protomylonitic fabric that lacks penetrative Rf/~b and microstructural estimates of finite strain grain-size reduction. The L-tectonite fabric is defined indicate that the L-S-tectonites in the Mitchel Range largely by grain elongations accomplished by the intra- record a substantially larger flattening component and crystalline deformation mechanisms described above. higher strain magnitudes than the L-tectonites in the However, the bulk rock aggregate also is cut by discrete Waterman Hills. However, grain-scale finite strain is zones of high shear strain that we call microshear sur- only part of the total macroscopic strain that a shear faces. These zones are defined by finely recrystallized zone records. The main differences between the two grains of quartz and mica, and commonly link biotite fish shear zone segments can be explained by considering the into arrays. The microshear zones are as thin as 10-30 tectonic history and larger-scale deformation of the microns but generally become thicker and more abun- mylonitic fabric in the shear zone. dant as the intensity of the deformational fabric in- The difference in strain magnitude may reflect syn- creases. kinematic emplacement of the granodiorite in the Microstructural evidence for prolate strain is most Waterman Hills. The granodiorite cuts transposed com- readily observed in the YZ plane, on which elongate positional layering in mylonitic pre-Tertiary rocks but is quartz subgrains commonly are oriented parallel to the itself mylonitized. Further, the timing of rift-basin margins of the nearest feldspar porphyroclast and biotite evolution overlaps with the age of the granodiorite. grains show no preferred orientation. Microshear sur- Current age constraints indicate that the extensional faces in the YZ plane are short, discontinuous and form basins in this area were initiated by 24 Ma (W. J. Taylor an anastomosing network with only a weak preferred et al. unpublished a°AR/39Ar data) and extension con- orientation parallel to the Yaxis. On the XYplane, high- tinued until 18.5 Ma (Dokka 1989, Glazner et al. 1989). strain zones are more continuous and oriented at the This indicates that the granodiorite was emplaced during small angle to the X axis. Evidence for Y axis shortening extension and may have only recorded some of the strain in the XY plane includes: concentrations of mica on the that transposed compositional layering in its wall rocks. margins of feldspar porphyroclasts, and domino-style The contrast between constrictional vs plane strain shear displacements across microcracks (Fig. 6e). The may reflect differences in scale-dependent strain parti- longest shear surface traces occur on the XZ principal tioning. The only mesoscopic and macroscopic struc- plane where they form evenly spaced sets parallel to the tural feature in Waterman Hills tectonites is the increase X axis. in strain magnitude toward the detachment which does Chemical alteration and mylonitization are more not affect the constrictional character of the finite strain. penetrative in the granitic L-S-tectonites from the Mit- Therefore, the total macroscopic finite strain in this chel Range. Cryptocrystalline replacement products shear zone segment was accomplished at the grain scale. after feldspar occupy up to 60% of the rock volume and In contrast, the L-S-tectonites in the Mitchel Range are commonly form long tails in the pressure shadows of strongly affected by mesoscopic and macroscopic syn- porphyroclasts. Microcracks in feldspar porphyroclasts mylonitic folding. These folds include a broad spectrum record wide separations and are filled with fibrous sec- of geometries but all have hinges oriented subparallel to ondary quartz. the SW-plunging mylonitic stretching lineation (Figs. 3b Microstructures in the Mitchel Range tectonites & c). Poles to both mylonitic layering and axial surfaces suggest that no finite elongation occurred along the Y of mesoscopic folds define great-circle girdles with best- axis. On the XY plane, microcracks in feldspar occur fit fold axes that also plunge to the southwest (Figs. 3d either perpendicular to the X axis or as conjugate sets at &e). a high angle to the X axis. However, regardless of the The coaxial syn-mylonitic folds are classified accord- crack orientation, quartz fibers defining the displace- ing to their style and relationship to the mylonitic fabric. ment vector always are parallel to the X axis (Fig. 6f). In Type 1 folds are upright open folds of the mylonitic contrast, flattening of porphyroclasts by shear displace- layering (Fig. 8a). These folds typically are symmetric, ments across microcracks and narrowing of fiber bundles have interlimb angles greater than 90 ° and lack an away from widely separated porphyroclasts commonly associated upright axial-planar . Type 2 folds were observed on the XZ plane. Lastly, although the YZ are strongly asymmetric, have interlimb angles less than plane displays a well defined foliation, no pressure 90 ° and contain an inclined axial-planar mylonitic folia- shadows were observed around the porphyroclast mar- tion that cuts through the short limb of the fold pair (Fig. gins. Therefore, microstructures that demonstrate 8b). Type 3 folds include recumbent isoclinal folds to shortening along the Z axis and extension along the X which the mylonitic foliation is axial planar (Fig. 8c), axis are absent along the Y axis, corroborating the Rf/~ and sheath folds which are much less common (Fig. 8d). Constrictional strain in a non-coaxial shear zone

Fig. 6. Rock slabs and photomicrographs cut subparallel to the principal planes of finite strain. (a) & (b) Granodioritc fmra the Waterman Hills, sample W- 11. Quartz is gray, biotite is black and feldspar is white, Note isotropic character of nlylonitic fabric in YZ plane (a) (scale bar = 5 ram). (c) & (d) Peralurainous granite from the Mitchel Range, sample M-7. Strain markers are altered garnet porphyroclasts (black), feldspar is white and quartz if light gray. Note well-developed planar fl)liation in YZ plane (c) (scale bar--5 ram). (e) XY plane of Waterman Hills L-tectonite, sample W-1 I. X axis extension is defined by quartz-filled tensile cracks. Y axis contraction is defined by shear displacement across raicrocracks, and by concentrations of recrystallized biotite on margins of feldspar porphyroclasts that lie perpendicular to the Z axis (scale bar-- 1.0 tara). (f) XY plane of Mitchel Range L-S-tectonite. sample M-5. Fibrous quartz (q) defines tensile separation of feldspar porphyroclasts (f). No evidence for Y axis strain (scale bar - 1.0 ram).

563 J. M. FLETCHER and J. M. BARTLEY

Fig. 8. Syn-mylonitic fold geometries in the Mitchel Range. (a) Type 1 upright gentle fold of mylonitic layering. (b) Type 2 recumbent asymmetric fold with axial planar cleavage cutting short limb of the fold pair. (c) Type 3 recumbent tight fold. Mylonitic foliation is the axial planar clcavage. (d) Type 3 sheath fold. Aspect ratio of fold closure is 2~:L Arrows point to surfaces containing the stretching lincation.

564 Constrictional strain in a non-coaxial shear zone 565

Table l. Strain data from the Waterman Hills and Mitchel Range. Principal stretches were calculated assuming constant volume strain. Data averages are arithmetic means except for K values. Average K values were calculated by fitting a straight line through the origin (l,1) and minimizing deviations in R~v. Sample locations are shown in Figs. 4 and 5

Sample R o. R,': R.,z K S,. S v S:

Waterman Hills Wl 2.64 1.09 2.88 18.22 1.97 (t.74 0.68 W2 3.42 1,01 3.45 242.00 2.28 0.67 0.66 W3 2.26 1.78 4.02 1,62 2.09 0.92 0.52 W4 2.86 1,28 3.66 6.64 2.19 (I.76 0.60 W5 2.94 1.26 3.70 7.46 2.22 0.75 0.60 W6 2.45 1.74 4.26 1.96 2.19 0.89 0.51 W7 3.58 1.20 4.30 12,90 2.49 0.69 0.58 W8 2.84 1.76 5.(10 2,42 2.42 0.85 0.48 W9 3.44 1.80 6.19 3,(15 2.77 0.81 0.45 WI0 4.16 1.24 5.16 13.17 2.78 (I.67 (t.54 Wll 5.22 1.10 5.74 42.40 3.11 (t.60 0.54 W12 4.59 1.96 9.00 3.74 3.46 {).75 0.38 W13 4.19 1.83 7.67 3.84 3.18 {L76 0.41 Average 3.43 1.47 5.110 6.01 2.55 0.76 (}.54

Mitchcl Range MI 7.45 5.17 38.52 1.55 6.60 0.89 {).17 M2 3.53 4.27 15.07 0.77 3.76 1 .{17 (t.25 M3 7.68 6.26 48.08 1.27 7.17 (t.93 0.15 M4 5.77 4.45 25.68 1.38 5.29 0.92 0.21 M5 4.28 5.38 23.03 (t.75 4.62 1.08 0.20 M6 5.45 5.55 30.25 (}.98 5.48 1.01 O. 18 M7 5.45 4.3(I 23.44 1.35 5.04 tl.92 0.21 M8 5.26 4.29 22.57 1.29 4.91 0.93 0.22 Average 5.61 4,96 28.33 1.11 5.36 0.97 (I.20

trace of a shorter wavelength type 1 synform which suggests that the brittle detachment contains antiformal Apparent and synformal undulations that are also broadly con- Constriction / gruent with the shorter wavelength type 1 folds in the 6. shear zone (Fig. 5b). These cross-cutting relationships oo indicate that much of the type 1 folding occurred in the / K:11jy ductile regime but we speculate that the undulations of 5' / // //. 4-* the detachment reflect the continued growth of type 1 folds in the brittle regime. R xy el " o //o 4 Mechanisms by which folds develop with axes parallel ,' /~ • / .~ /ff~\~'~ Apparent to the stretching lineation in shear zones can be divided .jK = o.u J Flattening into two main categories. The first involves progressive .7" rotation of fold axes that nucleate at high angles to the stretching lineation. Alternatively, folds may form with 2 ,1 ~ j • Waterman Hills (n=13) o Mitchel Range (n__.8) axes that initially lie subparallel to the stretching linea- tion. Cobbold & Quinquis (1980) provided theoretical I I I I I I and experimental models for fold nucleation and hinge- 2 3 4 5 6 7 line rotation during progressive simple shear. Their Ryz model 1 has become a widely accepted explanation of Fig. 7. Flinn diagram of Rflcp strain data, the formation of sheath folds and distribution of fold hinges in shear zones. However, several observations Sheath folds typically have hinge angles less than l0 ° and limit the application of this model to the Mitchel Range eye-shaped closures with aspect ratios of 2-4 in down- shear zone. (1) Open type 1 folds which are abundant plunge sections. and appear to record the initial stages of fold formation The dominant macroscopic structure of the shear zone could not have undergone significant hinge-line ro- in the Mitchel Range is a range-scale type 1 antiform that tation. (2) Fold hinges plot a cluster and do not form the contains smaller wavelength (approximately 2 kin) type predicted great-circle distribution. (3) Skjernaa (1989) 1 folds (Fig. 5). The brittle detachment cuts folded demonstrated that sheath folds with hinge angles less mylonitic layering but it is also broadly warped into a than 20 ° should have aspect ratios of 35:1 if formed by similar range-scale antiform (Fig. 5b). In the northwest this mechanism. Clearly, this prediction does not fit the part of the Mitchel Range, occur near the axial observed sheath fold geometries in the Mitchel Range. 566 J.M. FLETCHERand J. M. BARTLEY

Type I Type 2 Type 3 Fig. 9. Interpreted sequential development of folds as defined by the three styles of folds obscrved in the Mitchel Range. Type I folds nucleated with axes subparallel to the stretching direction in response to Y axis shortening. With continued shearing, type 1 folds developed into type 2 and 3 folds as outlined by Skjernaa (1989).

Skjernaa (1989) modeled the consequences of over- 'thrust' shear which is highly implausible given the lack printing horizontal simple shear on upright open folds of independent evidence for wrench shear. (2) Although with axcs subparallel to the slip vector. The predicted strain is heterogeneous in the Mitchel Range, no consist- fold geometries and hinge distributions match well the ent lateral strain gradient was observed. The abrupt observed structures in the Mitchel Range shear zone. decrease in strain magnitude between the Mitchel According to her model, if the upright progenitor fold Range and Waterman Hills is only an apparent gradient axis is parallel to the slip vector, the top portion of the due to synkinematic emplacement of the granodiorite. fold will be displaced relative to the bottom but no (3) A lateral displacement gradient can not produce true change in fold geometry will occur (Skjernaa 1989, fig. constrictional strain as observed in the Waterman Hills 5b). However, if the progenitor fold axis lies in the slip because the resultant simple shear is, by definition, plane but oblique to the slip direction, subsequent plane strain. shearing produces recumbent asymmetric folds similar Syn-mylonitic folds and fabrics in the Mitchel Range to type 2 and 3 folds (Skjernaa 1989, fig. 5c). For a given are likely to have formed in response to the partitioning shear strain, the resultant fold geometry becomes tighter of constrictional strain into plane strain at the grain scale as the initial angle between fold axis and slip direction and Y axis shortening through larger-scale folding. The increases. Initial angles less than 15 ° are required to range of coaxial fold geometries is considered to rep- create type 2 folds when a shear strain of 10 is applied resent a sequence in a continuous deformational process (Skjernaa 1989). Finally, sheath folds with geometries (Fig. 9). This process begins at the grain scale where similar to those seen in the Mitchel Range are formed deformation is dominated by simple shear. As the when the progenitor fold axis is oblique to the slip plane mylonitic fabric develops it is folded into type 1 folds. and rotates into the shortening field with subsequent The folds become asymmetric and tighter with sub- shearing (Skjernaa 1989, figs. 5e & f). sequent shearing as outlined by Skjernaa (1989). Field In the Mitchel Range type 1 folds have the same relationships indicate that a new axial-planar mylonitic geometry as progenitor folds of Skjernaa's (1989) model fabric develops as the short limb of an asymmetric fold is and field relationships indicate that they formed during rotated away from the XY plane of the shear zone. progressive shearing. Two possible mechanisms that These type 2 folds become progressively tighter until could create such folds include lateral displacement they are completely transposed. True Y axis shortening gradients (Coward & Potts 1983, Ridley 1986, Holds- only occurs in the earlier stages of fold formation be- worth 1990) or shortening along the Y axis of the shear cause final fold amplification takes place by movement zone in a constrictional strain regime. Lateral displace- on the axial planar mylonitic fabric that records plane ment gradients can be factorized into simple shear on strain. two orthogonal phmes with a common slip vector which This deformational process will produce the observed Ridley (1986) terms 'thrust- and wrench-shear' com- fold geometries and hinge distributions in the Mitchel ponents. The resultant strain is simple shear on an Range. The continuous nature of the process is reflected inclined plane that contains the common slip vector in the great-circle girdle distribution of poles to both the (Flinn 1979). Ridley (1986) demonstrates that folds will mylonitic foliation and the axial planes of the meso- form with axes nearly parallel to the stretching lineation scopic folds (Figs. 3d & e). Finally, lateral shortening in if the pre-existing layering lies parallel to the 'thrust- response to constrictional strain is consistent with the shear" plane. The fold axes and stretching lineation formation of L-tectonites in the Waterman Hills. initiate at 45 ° to the slip vector and progressively rotate into parallelism with it, The axial surfaces of these folds are parallel to the XYplane of the resultant simple shear DISCUSSION (Ridley 1986). Several factors render this mechanism of fold forma- We interpret the mylonites in the Mitchel Range and tion in the Mitchel Range unlikely. (1) The development Waterman Hills to record two distinct constrictional of upright folds requires ~wrench' shear to dominate strain paths. The L-tectonites in the Waterman Hills Constrictional strain in a non-coaxial shear zone 567

CRUSTAL-SCALE Y-AXIS ent-~ / SHORTENING Apparent ~'c~," o

6']' o

t 5-. / / / • o RxY4..1 , >~

• • / Apparent / Flattening 3-'./. , Fig. 11. Strain compatibility is maintained by Y axis shortening in the shear-zone bounding blocks. Possible structures include conjugate strikc-slip faults and folds with axes parallel to the movcment direc- oMitchel Range tion.

mation path at the grain scale may have evolved from I 2 3 4 5 6 7 constrictional to plane strain. Simple shear, requiring Ryz only one slip surface and one slip direction, may be Fig. 10. Effects of macroscopic strain and tectonic history on grain- favored over other more complex strain paths at the scale paths. Constrictional strain in the Waterman Hills is accom- plished at the grain scale by the formation of L-tectonites. Constric- grain scale. However, in the incipient stages of defor- tional strain in the Mitchel Range is accomplished by mcsoscopic and mation, the grain-scale deformational fabric probably macroscopic folding of L-S-tectonites about axes oriented subparallel will reflect the bulk strain regime because inherent to the stretching direction. Waterman Hills tcctonites rccord lower strain magnitudes because the syn-kinematic granodioritc only weaknesses in the rock are randomly oriented. For recorded the later fraction of the total strain history. example, microshear surfaces which define the L- tectonite fabric in the Waterman Hills appear to have accomplish non-coaxial constriction at the grain scale, nucleated on biotite grains and the altered portions of whereas constrictional strain in the Mitchel Range was feldspar porphyroclasts. In the initial stages of non- accomplished by movement-parallel folding of L-S- coaxial constriction, these inherent weaknesses can be tectonites that record plane strain at the grain scale (Fig. exploited in a manner that reflects the macroscopic 10). This interpretation raises several important ques- strain regime. However, at greenschist-facies conditions tions. Why do the shear zone segments accomplish the rock contains large concentrations of minerals with a constrictional strain by two completely different defor- single active slip system, e.g. quartz and phyllosilicates. mation paths? How is Y axis strain compatibility main- We hypothesize that as microlithons decrease in size and tained between the rocks in the shear zone and rocks minerals develop a preferred orientation, simple shear outside the shear zone? Is bulk constrictional strain on a single slip surface may become the dominant mode plausible in an extensional tectonic setting? of deformation at the grain scale. Finally, because the syntectonic granodiorite only Strain paths and deforrnational processes records the later stages of mylonitization, it is possible that the strain field changed from non-coaxial plane We consider three hypotheses to explain the develop- strain before granodiorite emplacement to non-coaxial ment of two distinct strain paths in the adjacent shear constriction after. Each of these three scenarios is con- zone segments. The pre-Tertiary rocks in the Mitchel sistent with our data. Range record a Mesozoic thermotectonic event and may have contained a pre-existing deformation fabric. The Strain compatibility Mesozoic tectonic layering may have controlled the grain-scale fabric of the superposed mylonitization. De- The ductile shear zone in the CMMCC records true pending on the initial orientation of the layering, it could constrictional strain and therefore cannot be bounded have acted as slip surfaces for simple shear or inhibited Y by rigid blocks, Instead, Y axis shortening must occur axis contraction. The Mesozoic fabric in this region throughout the crust in order to maintain strain compati- typically involves a strong subvertical gneissic layering, bility with the shear zone (Ramsay and Wood 1973, but on the grain scale it is statically recrystallized and Ramsay 1980). However, Y axis shortening in the hang- equigranular (Fletcher et al. 1991). Thus, although the ing wall block need not occur immediately above the early fabric may have controlled the character of mid- ductile shear zone but could affect rocks laterally offset Tertiary mylonitic strain at the grain scale, it should not from constriction in the shear zone. have affected the actual Rf/g) and microstructural esti- Y axis shortening in the fault-zone bounding blocks is mates of finite strain. likely to occur by formation of either: (1) folds with axes Alternatively, because the L-tectonites record lower parallel to the extension direction; or (2) networks of strain magnitudes than the L-S-tectonites, the defor- conjugate strike slip faults (Fig. 11). In the Colorado 568 J.M. FI.ETCHER and J. M. BARTLEY

ol-o'n x

Fig. 12. Formation of folds with axes parallel to the extension direction in an Andersonian stress regimc. Normal stress on a horizontal layer is equal to o~ and buckling in response to o2 is impossible. Normal stress on an inclined la~cr is less than ot and buckling m response to o. may be possible.

River Trough extensional belt, Yin (1991) documents tion of L-tectonites, movement-parallel folds and conju- folds with axes parallel to the movement direction in sills gate strike-slip faults. and mylonites in the footwall as well as in sedimentary However, on the scale of individual fault zones and rocks in the hanging wall. Detachment faulting in the tectonic blocks, the principal stress axes cannot coincide southern Basin and Range is commonly coupled with with the principal strain axes and still generate the conjugate strike-slip faults which together produce observed structures. When modeling the formation of large-scale constrictional strain (Wright 1976, Wernicke folds in the Colorado River detachment system, Yin etal. 1982, 1988, Anderson & Barnhard 1984, Glazner & ( 1991 ) demonstrated that it is possible to create buckle Bartley 1984). Therefore, we suggest that crustal-scale folds with axes parallel to the extension direction in a shear zones such as that in the CMMCC may not be subhorizontal elastic sheet with nearly uniaxial tectonic bounded by rigid blocks and consequently the finite stresses. Buckling instability for folds that form in re- strain need not behave according to the ideal shear zone sponse to ~2 compression will never develop because the model. normal stress across the elastic sheet is cq (Yin 1991). In fact, this relationship holds for other theologies as well. Dynamic feasibility of generating constrictional strain Smith (1977) showed that for Newtonian and non- and folding instability Newtonian media, end loads must be greater than nor- mal stresses to generate folding instability with reason- A nearly uniaxial with (o3 < o2 ~- 0~) is the able growth rates. simplest stress configuration that can produce constric- In the extensional tectonic regime, such a stress con- tional strain. Theoretical considerations suggest that figuration can occur if the fault zone is inclined to the such a stress field is reasonable in an extensional tectonic principal stress axes, i.e. the resolved normal stress setting. In the absence of imposed tectonic stresses, across the zone will be less than Ol and cr~ (Fig. 12). McGarr (1988) proposed that a hydrostatic state of Therefore, folding instability for folds with axes parallel stress (o~ = o2 = o3 = lithostatic load) is the most stable to the extension direction may be possible in the fault configuration in the crust. According to the Anderson zone, as well as in any inclined layering in the extending fault theory, crustal extension occurs when compressive . Additionally, the fault zone must be inclined to stresses are sufficiently reduced in one horizontal direc- the principal stresses to generate shear stress that drives tion along the 03 axis. Near the surface of the Earth, o! displacement. Even when the formation of movement- will be vertical and equal to the lithostatic load. How- parallel folds is mechanically impossible, Yaxis shorten- ever, the magnitude of c~, which is horizontal and ing by conjugate strike-slip faulting could occur in the perpendicular to the extension direction, should also uniaxial stress field because (oz - o~), the stress differ- remain approximately equal to the lithostatic load. ence that drives strike-slip faulting, is identical to Nearly uniaxial stresses are also commonly inferred (ol - o3), the stress difference that drives normal fault- from inversions of fault-slip and earthquake focal mech- ing, anism data in many extensional throughout the Basin and Range (Zoback 1989). Therefore, the principal stress axes crudely coincide with the principal strain axes on the scale of the entire CONCLUSIONS extensional orogen. The crust is simultaneously ex- tended along the subhorizontal 03 axis and shortened (l) Non-coaxial constriction in the CMMCC was along the subvertical 0~ axis by displacement on the accomplished by two different deformation paths. The detachment fault system. Additionally, the crust is first path accomplished constrictional strain at the grain shortened along the subhorizontal 02 axis by the forma- scale through the formation of L-tectonites. The second Constrictional strain in a non-coaxial shear zone 569 path involved a combination of plane strain at the grain Fletcher, J. M., Martin, M. & Bendixen, J. 1991. Mid-Tertiary mylonitization and deep-seated Mesozoic contraction in the Buttes scale and Y axis shortening through syn-mylonitic fold- (abs.). Cordilleran Section, Geol. Soc. Am. Abs. w. Prog. 23, A25. ing. Flinn, D. 1979. The deformation matrix and the deformation ellipsoid. (2) Syn-mylonitic folding in a shear zone that accom- J. Struct, Geol. 1,299-308. Glazner, A. F. & Bartley, J. M. 1984. Timing and tectonic setting of modates constrictional strain is characterized by the Tertiary low-angle normal faulting and associated magmatism in the nucleation and amplification of folds with axes parallel southwestern United States, 3, 385-396. to the stretching lineation. Folds nucleate as upright Glazner, A. F., & Bartley, J. M. 1991, Volume loss, fluid flow and state of strain in extensional mylonites from the central Mojave open folds and continued shearing produces recumbent Desert, California. J. Struct. Geol. 13, 587-594, tight and isoclinal folds as well as sheath folds. Glazner, A. F., Fletcher, J. M., Martin, M. W,, Walker, J. D. & (3) Corrugations in the brittle detachment of the BartIey, J. M. 1992. Widespread Miocene plutonism and ductile deformation (abstract). Eos 73, 548. CMMCC record Y axis shortening in the brittle regime Glazner, A. F,, Bartley, J. M. & Walker, J. D 1989. Magnitude and and do not appear to be primary undulations in the fault significance of Miocene crustal extension in the central Mojave surface. Desert, California. Geology 17, 50-53. Hamilton, W. 1988. Detachment faulting in the Death Valley region, (4) Y axis shortening outside the fault zone may be California and Nevada. Bull. U.S. geol. Surv. 1790, 51-85. accomplished by structures such as conjugate sets of Holdsworth, R. E. 1990. Progressive deformation structures associ- strike slip faults and folds with axes parallel to the ated with ductile thrusts in the Moine , Sutherland, N. Scotland. J. Struct. Geol. 12, 44.3452. extension direction. John, B. E. 1987. Geometry and evolution era mid-crustal extensional (5) A nearly uniaxial stress field ((73 ,~ (72 ~ (71) could fault system: Chemihuevi Mountains, southeastern California. In: generate constrictional strain and most of the structures Continental " (edited by Coward, M. P., Dewey, J. F. & Hancock, P. L.). Spec. Pubis geol. Soc. Lend. 28, associated with large-magnitude extension in the Basin 313-335. and Range. Based on independent theoretical consider- Kiser, N. L. 1981. Stratigraphy structure and metamorphism in the ations, we suggest that uniaxial stresses may arise in Hinkley Hills, Barstow. California. Unpublished M,S. thesis, Stan- ford University. extensional tectonic settings. McFadden, B. J., Swisher, C. C., Updykc, N. D. & Wood 1990. Paleomagnetism, geochronology, and possible tectonic rotation of Acknowledgements--This project was funded by NSF grants the middle Miocene Barstow Formation, Mojave Desert, Southern EA R8816944 and E A R8916838. Fortran programs written by Adolph California. Bull. geol. Soc. Am. 102, 478-493. Yonkee were used throughout the quantitative strain analysis. Stereo- Martin, M. W. & Walker. J. D. 1991. Upper Precambrian-Paleozoic plots were generated with Richard AIImendinger's program Stereonet paleogeographic reconstruction of the Mojave Desert, California. 4.5a. 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