Download Large-magnitude continental extension: An example

Large-magnitude continental extension: An example from the
central Mojave metamorphic core complex
John M. Fletcher*
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112
John M. Bartley
Mark W. Martin* Isotope Geochemistry Laboratory and Department of Geology, University of Kansas,
}
Lawrence, Kansas 66045
Allen F. Glazner Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599
J. Douglas Walker Isotope Geochemistry Laboratory and Department of Geology, University of Kansas,
Lawrence, Kansas 66045
ABSTRACT
The central Mojave metamorphic core complex is defined by a
belt of Miocene brittle-ductile extension and coeval magmatism.
The brittle-ductile fault zone defines a basin-and-dome geometry
that results from the interference of two orthogonal fold sets that
we infer to have formed by mechanically independent processes.
One fold set contains axes that lie parallel to the extension direction
of the shear zone and has a maximum characteristic wavelength of
about 10 km. The axial surfaces of these folds can be traced from
the footwall mylonites, through the brittle detachment, and into
hanging-wall strata. However, folds of mylonitic layering have
smaller interlimb angles than those of the brittle detachment, suggesting that the folds began to form during ductile shearing and
continued to amplify in the brittle regime, possibly after movement
across the fault zone ceased. Mesoscopic fabrics related to the
transport-parallel fold set indicate that the folds record true crustal
shortening perpendicular to the extension direction. We interpret
these folds to form in response to elevated horizontal compressive
stress perpendicular to the extension direction and suggest that this
stress regime may be a natural consequence of large-magnitude
extension.
The other fold set has axes perpendicular to the extension
direction and a characteristic maximum wavelength of about 50 km.
Mesoscopic fabrics related to these folds include northwest-striking
joints, kink bands, and en echelon tension-gash arrays. These fabrics formed after mylonitization and record both layer-parallel extension and northeast-side-up subvertical shear. The postmylonitic
fabrics are kinematically compatible with rolling-hinge-style isostatic rebound of the footwall following tectonic denudation.
The relative timing of extension-related magma intrusion and
ductile deformation varies through the central Mojave metamorphic core complex. On the scale of the small mountain ranges that
make up the central Mojave metamorphic core complex, no correlation was observed between either shear zone thickness or intensity
*Present address: Fletcher: Departamento de Geologia Centro de Investigación Cientı́fica y Educación Superior de Ensenada, B.C. Ensenada,
B.C., Mexico; Martin: Servicio Nacional de Geologia y Mineralia-Chile,
Grupo de Geologia Regional, Avda. Santa Maria 0104, Casilla 1347, Santiago, Chile.
of ductile deformation and either the proximity or relative volume
of extension-related igneous rocks. This suggests that models that
invoke a single upper-crustal genetic relationship, such as magmatism triggering extension or vice versa, do not apply to the central
Mojave metamorphic core complex.
Systematic variation in the relative timing of dike emplacement and mylonitization suggests that, at the time of dike emplacement, rocks in the Mitchel Range were below the brittle-ductile
transition while those in the Hinkley Hills were above it. The Hinkley Hills and Mitchel Range are separated by ;2 km in the dip
direction of the fault zone, which suggests that the vertical thickness of the brittle-ductile transition probably was between 100 and
950 m.
INTRODUCTION
Brittle-ductile faulting as recorded in Cordilleran metamorphic
core complexes are now recognized throughout the world as an
important mechanism of extension in previously overthickened continental crust. Examples include late- to post-Caledonian rifting on
the western margin of Norway (Norton, 1986; Andersen and
Jamtveit, 1990), extension in the Andean magmatic arc (Mpodozis
and Allmendinger, 1993), syncontractional spreading on the Tibetan plateau (Burg et al., 1984; Burchfiel and Royden, 1985), collapse of the Cycladic blueschist belt in the central Aegean Sea (Lee
and Lister, 1992), and extension of continental crust at the tip of a
propagating oceanic rift in the Solomon Sea (Davies and Warren,
1988; Hill et al., 1992; Baldwin et al., 1993). Additionally, spreading
at some mid-ocean ridges seems to involve such fault zones, based
on the results from ocean drilling (e.g., Cannat, 1987; Cannat et al.,
1987; Dick et al., 1987a) and the study of ophiolites (e.g., Varga and
Moores, 1985; Harper, 1985).
Much of the work on metamorphic core complexes in the past
decade has centered on three main areas: (1) understanding relationships between magmatism and extensional deformation, (2) understanding the characteristically nonplanar geometry of the lowangle fault zones, and (3) understanding mechanics of low-angle
normal faulting. This paper addresses issues concerning each of
these areas.
In many metamorphic core complexes, crustal extension occurred during igneous activity and genetic links therefore have been
GSA Bulletin; December 1995; v. 107; no. 12; p. 1468 –1483; 13 figures.
1468
LARGE-MAGNITUDE CONTINENTAL EXTENSION
inferred between the two processes. Elevated mantle heat flow
and/or the emplacement of mafic magma at the base of the crust has
been proposed to cause thermal softening, triggering extension (e.g.,
Rehrig and Reynolds, 1980; Sonder et al., 1987; Wernicke et al.,
1987; Gans et al., 1989). Lister and Baldwin (1993) recently proposed a more intimate genetic relationship, suggesting that midcrustal magmatism triggers extension and controls the location of
the brittle-ductile transition in the crust. They suggest that midcrustal magmatism is responsible for many characteristic features of
core complexes, including ductile deformation of footwall rocks,
spatial patterns of cooling ages, and characteristic geometric features of the fault zones. Abundant extension-related intrusions and
excellent exposures in the central Mojave metamorphic core complex permit tests of such proposed genetic relationships.
Brittle-ductile detachment faults in core complexes characteristically define dome-and-basin structural topography that results
from interference of two orthogonal sets of upright folds (Spencer,
1982, 1984; Davis and Lister, 1988; Yin, 1991; Yin and Dunn, 1992).
Several models have been proposed to explain the origin of these
folds, and each makes specific predictions for (1) macroscopic geometry of layered rocks in the upper and lower plates, (2) mesoscopic finite strain recorded in the upper and lower plates, and (3)
relative timing of mylonitization, deposition of rift sediments, and
formation of the undulations. Rock fabrics related to these two fold
sets have important implications for the origin of the dome-andbasin fault geometry, as well as for the kinematics and dynamics of
large-magnitude extension.
This paper defines the areal extent and characterizes the rock
types and fabrics of a large-displacement extensional shear zone
exposed in the central Mojave Desert. Field relations and structural
data in this paper complement geochronologic data from the central
Mojave metamorphic core complex published separately by Walker
et al. (in press). Extension-related intrusive rocks are used to limit
the timing of deformation and to recognize and correlate Miocene
rock fabrics throughout the multiply deformed terrane. Field relations in the central Mojave metamorphic core complex also help to
characterize the brittle-ductile transition during crustal extension.
Geographic and Tectonic Setting
Miocene crustal extension and basin development are recognized in the central Mojave Desert from the Rodman Mountains to
the Buttes (;90 km along strike; Fig. 1). A single master detachment fault may underlie the entire belt. However, differences in
structural style and extension magnitude permit the belt to be divided into two main segments. The southeastern segment in the
Figure 1. Geologic map of the Miocene extensional belt in the central Mojave Desert, California (The Buttes to Rodman Mountains)
showing the mylonitic and nonmylonitic segments of the detachment fault system. Modified in part after Dokka (1989) and Walker et al
(in press); see text.
Geological Society of America Bulletin, December 1995
1469
FLETCHER ET AL.
Rodman Mountains and Newberry Mountains exposes no footwall
mylonites nor extension-related plutonic rocks. In contrast, the
northwestern segment of the fault system, exposed from the Mitchel
Range to the Buttes, is characterized by well-developed footwall
mylonites and widespread extension-related igneous intrusions.
Based on the offset of regional pre-Tertiary lithologic markers in the
central Mojave Desert, ;40 –50 km of northeast-directed displacement occurred across the northwestern brittle-ductile fault segment
(Glazner et al., 1989; Martin et al., 1993). In contrast, field relations
in the Rodman Mountains suggest that the southeastern fault segment accomplished significantly less displacement, which is consistent with the lack of mylonites (Bartley and Glazner, 1991, and unpub. field data). Martin et al. (1993) suggested that the boundary
between these two segments is defined by an unexposed right-lateral
fault system that transfers extension from the Colorado River
Trough to the central Mojave.
The two segments of the extensional terrane approximately
correspond to the Daggett and Waterman terranes of Dokka (1989).
However, Dokka (1989) also placed a terrane boundary (Waterman/
Edwards) within our northwestern fault segment, separating the
Buttes–Harper Lake area from the rest of the core complex. Much
of the Edwards Terrane also includes unextended or very weakly
extended crust in the western Mojave Desert (Bartley et al., 1990b).
We found no evidence that either the style or magnitude of crustal
extension changes between the Buttes–Harper Lake area and the
rest of the brittle-ductile fault segment.
The northwestern segment of the Mojave extensional belt has
been termed the central Mojave metamorphic core complex (Glazner
et al., 1989; revised by Fletcher and Bartley, 1994). This paper synthesizes and interprets extensional deformation and coeval igneous
intrusion in the footwall of the central Mojave metamorphic core
complex, which is exposed in three areas that we refer to as domains
(Fig. 2). The Mitchel domain includes the Waterman Hills and
Mitchel Range and provides the thickest exposures of the brittleductile fault zone, as well as the most intense mylonitic fabrics. The
Hinkley domain, which includes the Hinkley Hills and Lynx Cat
Mountain, contains exposures of the fault zone that are structurally
up-dip from the Mitchel domain. The Buttes domain, which includes the Buttes and surrounding hills, makes up the westernmost
exposures of the brittle-ductile detachment in the central Mojave
metamorphic core complex and lies along strike from the other
domains.
LITHOLOGIC AND STRUCTURAL CORRELATIONS
Rock Types
Pre-Tertiary Rocks. The ductilely deformed footwall of the
central Mojave metamorphic core complex is predominantly composedofpre-Tertiarymetaigneousandmetasedimentaryrocks.Metaigneous rocks are dominated by a plutonic complex made up of
quartz diorite, hornblende-biotite diorite, hornblende gabbro, and
subordinate two-mica granite. Metasedimentary wall rocks of this
igneous complex are composed of varying rock suites across the
central Mojave metamorphic core complex. In the Mitchel Range
and Hinkley Hills, feldspathic micaceous quartzite is the most abundant metasedimentary rock, with subordinate pelitic schist, orthoquartzite, bluish-gray graphitic calcite marble with siliceous stringers, white calcite marble, and tan dolomitic marble. Kiser (1981)
correlated this sequence with Vendian-Cambrian strata of the Cordilleran miogeocline. In the Buttes domain, marbles are virtually
identical to those in the Hinkley Hills and Mitchel Range, but the
metaclastic rocks are more calcareous and less aluminous. Amphibolitic carbonaceous quartzite is the most abundant metasedimentary rock type in this domain. Also, a significant amount of paraamphibolite is present here but is virtually absent in the other
Figure 2. Geologic map of the central
Mojave metamorphic core complex showing
the three domains of this study: WH and
MR make up the Mitchel domain; HH and
area to the immediate northwest make up
the Hinkley domain; B is the Buttes domain.
Area ornamented with wavy lines is alluvium-covered but inferred to be underlain by
Miocene mylonite. Abbreviations: B, Buttes;
CM, Calico Mountains; FP, Fremont Peak;
GF, Garlock Fault; GH, Gravel Hills; HH,
Hinkley Hills; IM, Iron Mountain; L, Lead
Mountain; LCM, Lynx Cat Mountain; LM,
Lane Mountain; MH, Mud Hills; MR,
Mitchel Range; SAF, San Andreas fault;
WH, Waterman Hills.
1470
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
domains. We interpret these lithologic variations to reflect differences in either age or facies of the sedimentary protolith.
Pre-Tertiary rocks variably contain the Tertiary mylonitic fabric
and a high-grade gneissic fabric probably related to Mesozoic contraction. Pelitic rocks preserve relict upper amphibolite-facies assemblages including garnet 1 biotite 1 muscovite 1 quartz 6 kyanite 6 sillimanite 6 staurolite. High-grade fabrics in the central
Mojave metamorphic core complex and vicinity appear to be of two
different ages, Late Cretaceous in the Buttes (Glazner et al., 1994;
M. W. Martin, unpub. data) and Late Jurassic in the Iron Mountains
(Boettcher and Walker, 1993). In each area the gneissic fabric is
steep and predominantly strikes northeast.
Miocene Igneous Rocks. The central Mojave metamorphic core
complex is intruded by several extension-related igneous phases.
U/Pb zircon data from all samples of these rocks define a single
discord with a lower intercept of 21.9 6 0.8 Ma, and some of the
calculated error is interpreted to reflect real variation in ages of the
suite of intrusive rocks (Walker et al., in press). Miocene granite
crops out in all three domains and may form a continuous batholith
at depth. Primary minerals in the medium-grained equigranular
granite include oligoclase, orthoclase, quartz, biotite, and minor
hornblende. Texturally, oligoclase is euhedral and cyclically zoned,
whereas orthoclase poikilitically overgrows all other primary minerals and is partially replaced by myrmekite.
Dikes in the central Mojave metamorphic core complex range
in composition from basalt to rhyolite, but the majority (60%– 80%)
are dacitic to rhyodacitic. Basaltic dikes were not analyzed by
Walker et al. (1995), but these dikes never contain a mylonitic fabric
and may be much younger, possibly related to Pliocene basalt flows
at Black Mountain (Fig. 1). Rhyolite dikes are generally aphanitic,
but some are porphyritic with 10% plagioclase, quartz, and rare
garnet phenocrysts. Dacite dikes are commonly porphyritic with
30%– 40% phenocrysts of plagioclase 1 quartz 6 biotite 6
hornblende.
A dacite dike in the Mitchel Range yielded a zircon U/Pb age
of 23 6 0.9 Ma (Walker et al., 1990), but dikes in the Hinkley Hills
have complex U/Pb systematics that are presently uninterpretable
(Walker et al., in press). However, the dikes in both areas are lithologically identical, and available geochronologic data at least permit
the possibility that they were intruded synchronously. If this is true,
field relations in the central Mojave metamorphic core complex can
be used to characterize the nature of the brittle-ductile transition, as
discussed below, because the relative timing of ductile deformation
and magma emplacement varies systematically between domains.
Deformational Fabrics
Synmylonitic Fabrics and Folds. In all domains, the mylonitic
stretching lineation plunges to northeast or southwest (Figs. 3A, 3E,
and 3I) and shear sense across the foliation is consistently northeast
directed (Dokka, 1989; Bartley et al., 1990a). The mylonitic fabric
forms L- and LS-tectonites and varies in intensity from protomylonitic to ultramylonitic. Protomylonitic rocks commonly are pure Ltectonites and rarely contain a measurable foliation. Mylonitization
involved transitional brittle-plastic deformation mechanisms (Bartley et al., 1990a; Fletcher and Bartley, 1994). Quartz and calcite
occur as mosaics of subgrains and neoblasts, which suggests that
dislocation creep was the dominant deformation mechanism. Feld-
spar is cataclastically deformed and hydrothermally altered; most of
the strain in feldspar is accomplished by the cryptocrystalline alteration product, perhaps by grain boundary sliding or alteration-induced dynamic recrystallization. Dolomite is typically cracked and
contains abundant glide twins. 40Ar/39Ar release spectra in a mylonitic pre-Tertiary quartzite limit peak temperatures of mylonitization to 300 – 400 8C because phengitic white mica yields a disturbed
spectrum at about 52 Ma and biotite is totally reset at 20 –22 Ma
(Wanda Taylor and John Bartley, unpub. data).
Mylonitic foliation is folded at all scales about an axis oriented
subparallel to the stretching lineation of the shear zone. In all three
domains, poles to the mylonitic foliation define a great circle girdle
with a best-fit fold axis oriented subparallel to the local stretching
lineation (Figs. 3B, 3F, and 3J). Fletcher and Bartley (1994) concluded that the mylonitic fabric records horizontal shortening perpendicular to the stretching lineation of the shear zone through the
formation of synmylonitic folds and L-tectonites. We demonstrate
below that these folds are related to northeast-trending upright
folds of the detachment fault. Variations in the character of mylonitization and synmylonitic folding across the central Mojave metamorphic core complex are documented below.
Postmylonitic Fabrics. Rocks in the footwall of the central Mojave metamorphic core complex contain two main classes of postmylonitic fabrics: joints and a composite fabric of kink bands and en
echelon tension-gash arrays (Figs. 3C, 3G, 3K, 3D, 3H, and 3L).
Both fabrics strike northwest, dip steeply, and are predominantly
found in rocks that were previously mylonitized. Joints are abundant
in all rock types. They occur as single and conjugate sets that are
oriented subperpendicular to the stretching lineation and commonly
accommodate millimeter-scale tensile and normal-sense offsets
(Fig. 4). Joints in the Mitchel Range are divisible into two groups
depending on the plunge of the stretching lineation. Where the
stretching lineation plunges to the southwest, joints mainly dip to
the northeast; where the stretching lineation plunges to the northeast, the joints dip to the southwest (Fig. 3C). This pattern suggests
that the joints formed before or during folding of the shear zone
about a northwest-rending axis and that the maximum extension
direction in the brittle regime crudely coincided with that in the
earlier ductile regime.
The composite kink-band and en echelon tension-gash fabric is
best developed in strongly layered rocks. The composite fabric is
common in ultramylonitic rocks of the mafic igneous complex and
in Miocene dikes but is rare in quartzite and marble. Tension-gash
arrays commonly occur within kink bands and may accommodate
dilation during kinking. However, tension-gash arrays and kink
bands each occur independently of each other. Planar tension
gashes are typically oriented 458–208 from the array boundary, which
suggests that, in some cases, they accommodated boundaryperpendicular extension as well as boundary-parallel simple shear
(cf. Ramsay and Huber, 1987).
In the Mitchel Range, the composite fabric is subvertical and
consistently shows northeast-side-up shear sense (Bartley et al.,
1990a). The fabric commonly is oriented subperpendicular to mylonitic foliation such that it accomplished neither layer-parallel
shortening nor extension, although some contractional kink bands
were observed in the Mitchel Range (Bartley et al., 1990a). The
cleavage typically is better developed in the hinge and southwestdipping limb of the antiformal arch. In the Buttes and Hinkley Hills,
Geological Society of America Bulletin, December 1995
1471
Figure 3. Equal-area stereoplots of Miocene deformational fabrics in ductilely sheared rocks from each domain. (A, E, I) Mylonitic
stretching lineation. Shaded boxes are maximum eigen values of the data sets. (B, F, J) Poles to mylonitic foliation. Shaded boxes are
minimum eigen values that define the best-fit macroscopic fold axis. In each subdomain the mean stretching lineation orientation is nearly
parallel to the best-fit macroscopic fold axis. (C, G, K) Poles to postmylonitic joints. Shaded boxes are maximum eigen values. In the
Mitchel Range, joints dip to the northeast in area where the stretching lineation plunges to the southwest and vice versa. (D, H, L) Poles
to the postmylonitic composite kink-band and tension-gash cleavage. Shaded boxes are minimum eigen values; n 5 number of samples.
1472
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
A
B
Figure 4. Postmylonitic deformational fabrics. (A) Outcrop photo
of conjugate joints showing small normal-sense shear offsets. (B)
Monoclinal kink bands and tension-gash arrays on saw-cut surface
of Miocene dacite from The Buttes. The bands and arrays are subperpendicular to the mylonitic foliation and accommodate northeast-side-up shear, but the two are not always spatially coincident.
many exposures of the composite fabric include conjugate sets, but
the northeast-side-up set is predominant. Displacement across conjugate sets of kink bands and tension gash arrays accomplishes layerparallel extension.
In summary, both the joints and the composite cleavage mainly
accommodate layer-parallel extension or northeast-side-up subvertical shear. We discuss below the implications of this pattern for
mechanics of the extension-perpendicular fold set.
DOMAINAL VARIATIONS IN MYLONITIZATION
AND MAGMATISM
Mitchel Domain
The extensional ductile shear zone in the Mitchel domain affected two rock assemblages. The Waterman Hills mainly are un-
derlain by the Miocene Waterman Hills Granite, whereas the
Mitchel Range is dominated by the heterogeneous pre-Tertiary
basement complex (Fig. 5A). The two assemblages are separated by
a klippe of Miocene volcanic and sedimentary rocks. Despite the
structural differences summarized below, both assemblages record
noncoaxial constrictional strain (Fletcher and Bartley, 1994).
In the Waterman Hills, finite strain increases structurally upward. Most of the shear zone is an outcrop belt, several hundred
meters wide, of protomylonitic L-tectonite that is transitional between undeformed granite in the structurally lowest exposures and
10 –20 m of ultramylonite beneath the detachment (Bartley et al.,
1990a; Fletcher and Bartley, 1994). LS-tectonites are rare and found
mainly in the ultramylonite. Finite strain in the protomylonitic Ltectonite is strongly constrictional, from which Fletcher and Bartley
(1994) inferred that the rocks record a significant amount (;75%)
of horizontal shortening perpendicular to the inferred transport direction of the shear zone.
The shear zone is thicker in the pre-Tertiary rocks of the
Mitchel Range than in the Waterman Hills granite and includes the
most intense mylonitization found in the central Mojave metamorphic core complex. Ultramylonite is exposed down to the lowest
exposed structural level, .1 km beneath the detachment. Compositional layering is fully transposed into the ultramylonite fabric.
Quartzite and marble units generally do not exceed 5 m in thickness,
but individual horizons commonly can be traced for 1– 4 km. The
shear zone contains weakly deformed lozenges up to 1 km2 in outcrop area, bounded by steep strain gradients that can be mapped as
contacts (Fig. 5A). Mylonite bounds the lozenges structurally both
above and below (Fletcher, 1994); therefore, the strain gradients do
not represent a ‘‘mylonite front’’ similar to that described in the
Whipple Mountains (e.g., Davis and Lister, 1988; Reynolds and
Lister, 1990).
LS-tectonites predominate in the Mitchel Range but L-tectonites also are common. The LS-tectonites record large-magnitude
plane strain at the grain scale (Fletcher and Bartley, 1994), but the
fabric is strongly folded about an axis oriented subparallel to the
extension direction of the fault zone. These nearly coaxial folds
range from upright and open to recumbent and isoclinal, but most
are close and asymmetric. Fletcher and Bartley (1994) argued that
this variation of fold styles represents an evolutionary sequence with
progressive mylonitization, from open folds initiated with hinges
nearly parallel to the transport direction toward recumbent isoclinal
folds. The initial upright folds are inferred to record the same subhorizontal shortening of the shear zone perpendicular to the extension direction that is recorded by constrictional strain in the Miocene granite. However, relationships in the other domains suggest
that other folding processes operated as well.
The detachment fault in this domain defines a macroscopic
upright antiform-synform pair with an axis subparallel to the extension direction and a wavelength of ;10 km (Fig. 5B). The synform
in the detachment fault coincides with a tight syncline in hangingwall strata. The antiform in the detachment coincides with the
range-scale antiformal culmination in mylonites of the Mitchel
Range. The mylonites also contain shorter wavelength (;2 km)
upright folds and, in the northwestern Mitchel Range, synformal
klippen are preserved near the hinge-surface trace of one of these
shorter-wavelength synforms (Fig. 5B). Antiformal and synformal folds of the detachment therefore broadly correspond to
Geological Society of America Bulletin, December 1995
1473
FLETCHER ET AL.
A
B
Figure 5. Geologic map and cross section of the Mitchel Range and Waterman Hills. (A) Geologic map of Mitchel Range and
Waterman Hills showing the brittle detachment, mylonitic fabrics in the ductile shear zone, macroscopic folds, and cross-section line. (B)
Simplified cross section of the fault zone in the Mitchel Range and Waterman Hills showing relationship of folds in the footwall mylonites
and hanging-wall strata to undulations of the brittle detachment. Cross-section line is perpendicular to the transport direction.
folds of more than one wavelength in both footwall mylonites and
hanging-wall strata. However, folds of the mylonites are tighter
than those of the brittle detachment (Fig. 5B). These relationships indicate that much of the upright folding occurred when
footwall rocks lay in the ductile regime. We interpret undulations
of the detachment to reflect continued fold growth in the brittle
regime.
Magmatism. Field relations suggest that extension-related igneous rocks in this domain were emplaced synkinematically. The
difference in strain magnitude between the Miocene granite and
1474
adjacent wall rocks suggests that the granite was emplaced synkinematically and only recorded the latter stages of mylonitization
(Fletcher and Bartley, 1994). Mylonitic fabrics are equally well developed in dacite dikes and their wall rocks, and, within the ultramylonites, the dikes are transposed into parallel with the mylonitic
fabric. Relations of rhyolite dikes commonly are similar, but some
dikes are less mylonitized and are oriented at high angles to the
mylonitic fabric. Some of the rhyolite dikes clearly were emplaced
synkinematically, but the transposed and highly deformed dikes may
have been emplaced either pre- or synkinematically. Based on tim-
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
A
B
Figure 6. Geologic map and cross section of the Hinkley Hills. (A) Geologic map of Hinkley Hills. (B) Cross section perpendicular
to the transport direction of the fault zone in the Hinkley Hills, showing truncation and transposition of older high-grade fabric by the
folded ultramylonite zone.
ing relationships in the Hinkley Hills discussed below, we suspect
that even the highly deformed dikes in the Mitchel Range were
emplaced synkinematically.
Hinkley Domain
The overall map pattern of the Hinkley domain matches that of
the Mitchel domain (Fig. 6A), with pre-Tertiary rocks on the south
and Miocene granite on the north, separated by a klippe of Tertiary
strata preserved in a synform in the detachment surface (Fig. 2).
Removing roughly 5 km of right slip across the Harper Lake Fault
brings the three matching rock suites in the two domains into contiguity (Fig. 2). Therefore, we interpret the Hinkley domain to be
the up-dip continuation of the fault zone in the Mitchel domain.
Although the rock assemblages are virtually identical in the two
domains, the character of deformation and relative timing of dike
emplacement differ markedly.
Miocene shear strain is heterogeneous in the Hinkley Hills such
that an older, probably Mesozoic, sillimanite-grade fabric is well
preserved in much of the area. The older fabric generally strikes
northeast, dips steeply, and is coarser grained than the Miocene
mylonitic fabric; however, it can be difficult to distinguish the two
Geological Society of America Bulletin, December 1995
1475
FLETCHER ET AL.
the shallow mylonitic fabric on the steep northeast-striking gneissic
fabric. However, such transposition will not produce the upright
folds or asymmetric folds with interlimb angles $908, which we infer
to have formed in response to horizontal shortening perpendicular
to the extension direction.
Magmatism. Miocene granite crops out poorly in the low hills
and pediment north of Mount General (Fig. 2), where the granite
contains a mylonitic fabric and thus was emplaced pre- or synkinematically. Miocene dikes make up about 20% of exposures in the
Hinkley Hills. The dikes generally are more altered than those in the
Mitchel Range but are otherwise petrographically identical to dikes
elsewhere in the central Mojave metamorphic core complex. The
dikes typically strike west-northwest and dip moderately to steeply,
but gently dipping sills injected along the mylonitic layering also are
observed. Nearly all dikes crosscut the mylonitic fabric, although a
few dikes on the eastern edge of the Hinkley Hills contain the mylonitic fabric. These relations indicating postmylonitic dike intrusion
contrast strongly with relations in the Mitchel Range. However, all
of the dikes are truncated by the brittle detachment on the flanks of
Mount General (Fig. 6A), and therefore these dikes must have been
emplaced during extension.
The Buttes Domain
Figure 7. Stereoplots of mesoscopic fold-related fabrics in the
Mitchel Range and Hinkley Hills. Fold hinges plot as a cluster that
is subparallel to the stretching lineation of the shear zone. Poles to
hinge surfaces define a great circle girdle suggesting that they are
also folded about macroscopic axes that are nearly parallel to the
stretching lineation. Abbreviation: n 5 number of samples.
fabrics in the field. A prominent ultramylonitic shear zone is approximately 30 –100 m thick and contains several upright antiforms
and synforms with an axis oriented subparallel to the extension direction (Fig. 6B). Complex drag folds with inclined hinge surfaces
define the intersection between the steep high-grade fabric and the
gently dipping ultramylonite zone. In general, dolomitic marble and
quartzofeldspathic rocks are sharply truncated, but quartzite and
calcite marble are transposed into parallelism with the shear zone.
Quartzite and calcite marble layers are attenuated, typically by
50%–90%, and we infer that these are the least competent rock
types in the area. LS-tectonites predominate in the ultramylonitic
shear zone. Brittle faults at a low angle to the ductile fabric also are
common, however. Therefore, even on the macroscopic scale, the
shear fabric in the footwall shows transitional brittle-ductile
characteristics.
Mesoscopic folds in the ultramylonite zone are coaxial with the
stretching lineation (Fig. 7) and display the same spectrum of geometries as that in the Mitchel Range. The large-scale drag folds in
the area imply that some recumbent folds formed by overprinting of
1476
The Buttes domain forms the westernmost known exposure in
the central Mojave Desert of rocks that record Miocene brittleductile extension. The dominant rock fabric in the region is a
Mesozoic gneissic fabric that we interpret to have developed during
migmatization of pelitic rocks. A leucosome layer from the migmatite yielded a concordant U/Pb monazite age of 94.8 6 1.0 Ma
(M. W. Martin, unpub. data). This fabric is variably overprinted by
Miocene mylonitization along discrete shear zones that generally
are 5–20 m thick. However, in the central and southern Buttes, the
main mylonitic shear zone is at least 70 m and 200 m thick, respectively (Fig. 8). In neither area is upper shear zone boundary exposed.
Thinner shear zones located structurally below the thick shear-zone
exposures are interpreted to be branches off of the main extensional
shear zone.
It is uncertain whether the brittle detachment is exposed in the
Buttes. The best candidate for such a structure occurs in the western
Buttes (Dokka et al., 1988, 1991; Dokka, 1989; Fig. 8). The fault dips
shallowly and contains southwest-plunging slickensides which are
consistent with the orientation of the mylonitic stretching lineation.
However, the footwall contains only a 5- to 20-m-thick cataclastic
zone and no mylonite, and thus it differs from any other exposure
of the detachment in the central Mojave metamorphic core complex. The fault separates Mesozoic mafic plutonic rocks in the footwall from medium-grained granite in the hanging wall. Neither the
sense nor amount of offset across the fault is clear, but the hangingwall granite has not been recognized elsewhere in the footwall of the
central Mojave metamorphic core complex and could represent fartraveled basement. However, the granite resembles similar granite
that is abundant west of the Buttes domain; if correct, this correlation does not delimit well the slip magnitude across the fault.
Another fault exposed in the Buttes domain strikes northwest
and dips about 308 to the northeast. Based on offset of a macroscopic
fold hinge in the northern Buttes (piercing point in Fig. 8), this fault
accommodated ;600 m of right-normal oblique slip.
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
Figure 8. Geologic map of the Buttes.
In the Buttes domain, it is difficult to distinguish mesoscopic
folds related to mylonitization from those related to the earlier
high-grade deformation. However, the two macroscopic folds
mapped in the area may have formed during Miocene mylonitization. In both folds, the mean orientation of the mylonitic foliation
lies between the orientations of the fold limbs and approximately
contains the fold axis. Therefore, the mylonitic fabric could represent the axial-planar cleavage of the folds (Fig. 9). In the synform in
the southern Buttes, the entire southern limb is a mylonitic shear
zone, whereas the northern limb is defined by the high-grade fabric.
In the northern Buttes, the mylonitic fabric is best developed in the
north-striking limb. Therefore, we interpret these folds to have resulted from localized shear and transposition of the high-grade fabric into parallelism with mylonitic shear zones.
Magmatism. Similar to the Mitchel Domain, Miocene granite
in the Buttes Domain contains a pronounced strain gradient with
structural level. Granite at the bases of most of the tors (;75 m of
relief) in the central Buttes is either undeformed or protomylonitic
Geological Society of America Bulletin, December 1995
1477
FLETCHER ET AL.
with an L-tectonite fabric. This fabric progressively grades into SCmylonite on most of the summits. The pluton-wall rock contacts are
not well exposed, and detailed relative timing relations could not be
determined. Dacite and felsite dikes show mutually crosscutting
relations with the shear zones, suggesting that emplacement occurred both before and after mylonitization. In some cases, dacite
dikes in the Buttes contain a wall-parallel mylonitic fabric that
affects the country rock only within 0.1–1.0 m of the contact. Such
strain localization could have occurred as a result of thermal
weakening of wall rocks during dike emplacement or hydrolytic
weakening caused by wall-rock hydration along the dikes. The
relations suggest that deformation may have overlapped in time
with emplacement.
DISCUSSION
Basin-and-Dome Geometry of Detachment Faults
The master low-angle normal faults of metamorphic core complexes typically are affected by two orthogonal sets of upright folds
that have different characteristic wavelengths and accomplish different types of finite strain. Extension-parallel folds have a maximum characteristic wavelength of about 10 –20 km. Extension-perpendicular folds typically include a single antiform-synform pair
with a maximum characteristic wavelength of about 50 –100 km. In
the central Mojave metamorphic core complex, both fold sets also
include shorter-wavelength folds. We infer these two fold sets to
result from independent mechanical processes.
Extension-Parallel Folds. In the central Mojave metamorphic
core complex, several lines of evidence suggest that the extensionparallel folds formed during active displacement across the fault
system and record horizontal shortening perpendicular to the extension direction. The folds are observed in upper-plate strata, in
the brittle detachment, and in lower-plate mylonites. The hingesurface traces of the upright folds spatially coincide in all of the
folded layers. However, in the Mitchel Range and elsewhere (e.g.,
Davis and Lister, 1988; Mancktelow and Pavlis, 1994), macroscopic
folds in the mylonites have smaller interlimb angles than the undulations in the detachment. We interpret this relation to indicate
that the folds began forming in the ductile regime and continued to
amplify in the brittle regime. In the central Mojave metamorphic
core complex, ductile shearing followed two distinct constrictional
strain paths that accomplished horizontal shortening perpendicular
to the extension direction (Fletcher and Bartley, 1994): either directly at the grain scale to form L-tectonites or by a combination of
plane strain at the grain scale and synmylonitic folding. In other
Cordilleran core complexes, the youngest rift-related sediments are
also folded (e.g., Compton, 1975; Yin and Dunn, 1992; Duebendorfer and Simpson, 1994), which suggests that shortening perpendicular to the extension direction may have outlasted displacement
across the fault zone.
Previously proposed genetic models for the upright extensionparallel folds can be divided into three types: (1) buckling in response to increased horizontal compression normal to the extension
direction, (2) bending in a heterogeneous vertical stress field, or (3)
the folds reflect original topography on the fault surface rather than
deformation of a previously more planar surface (Fig. 10). Only the
first type of model can cause true crustal shortening during active
1478
Figure 9. Stereoplots of poles to mylonitic foliation and compositional layering in the Buttes showing relationships between orientation of macroscopic folds and local mylonitic fabric. In both
areas of the Buttes, the mean orientation of the mylonitic foliation
contains the macroscopic fold hinge, which suggests that the mylonitic foliation is the axial planar fabric.
displacement. Yin (1991) showed that buckling of a horizontal elastic sheet only can occur if the horizontal stress is greater than the
vertical normal stress. Because the maximum normal stress ordinarily is vertical in an extensional regime, buckling of horizontal
layering should be suppressed. If extension-parallel folds form by
buckling, some process must decrease the ratio of the normal stress
acting across the layering to the horizontal compressive stress perpendicular to the extension direction. Because fold orientations consistently parallel the extension direction, as it varies along the Cordillera (e.g., Wernicke, 1992), we infer that the stress-modifying
process must be inherent in the dynamics of large-magnitude
extension.
Fletcher and Bartley (1994) suggested that the normal stress on
fault zone mylonites (or any other layered medium) may be less than
the lithostatic load if the layers are not horizontal. Buckling instability thus could arise if the horizontal compressive stress were equal
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
and produce shortening within the denuded region and extension
along the margins or transfer zones (Fig. 11C). Alternatively, the
margins of the extended terrane could move laterally inward and the
associated extension in this lateral dimension could occur in crust
that is far removed from the region of extension.
Variations in vertical stress in the plane perpendicular to the
extension direction could arise from (1) differential tectonic unroofing between the extended terrane and its lateral margins (Spencer,
1982, 1984) or (2) buoyant forces associated with uncompensated
undulatory crustal roots or lower-density plutons (Yin, 1991). Although differential unroofing between the extended terrane and its
unextended margins is an important source for vertical stress heterogeneities, this unroofing should produce folds with wavelengths
of the order of the lateral dimension of the extended terrane, that
is, greater than the generally observed 10 –20 km. Yin (1991) showed
that it is mechanically feasible to create folds with the observed
maximum characteristic wavelength, given a grid-like network of
uncompensated crustal roots or plutons with the proper size and
spacing. However, whether such a configuration of uncompensated
Figure 10. Schematic cross sections of core-complex fault
zones, drawn perpendicular to the extension direction, showing the
three main classes of proposed genetic models for extensionparallel folds/undulations. See text for explanation.
to or even less than the lithostatic load, depending on the inclination
and material properties of the layered medium.
Mechanical effects of tectonic denudation may also encourage
extension-perpendicular buckling. Tectonic denudation should not
only reduce the vertical normal stress on the remaining rocks, but
also may increase horizontal stress. In laterally homogeneous and
confined lithosphere, horizontal stress increases as some function of
the vertical stress (rgh); in time-relaxed viscous crust, horizontal
stress equals vertical stress (Fig. 11A). Prior to denudation, the
integrated horizontal force from this stress profile is supported by
the full crustal thickness. However, if the lower crust is weak, such
that the crust is decoupled from the upper mantle, then, after tectonic denudation, the horizontal stress in the denuded region must
increase because the same horizontal force acts over a smaller crosssectional area (Fig. 11B).
Isostatic rebound of the footwall will reduce the gravitational
potential gradient along the margins of the extended terrane. However, if isostatic compensation occurs by lateral flow of midcrustal
material with a low effective viscosity, then the upper crust may be
mechanically decoupled from the lower crust and mantle (Block and
Royden, 1990; Wernicke, 1990), and the horizontal force, generated
by the topographic gradient, will be distributed only in the extended
upper crust. Sediment-transport and facies patterns in the main rift
basin of the central Mojave metamorphic core complex suggest that
significant lateral highlands existed west of Fremont Peak and east
of Lead Mountain (Fillmore and Walker, in press; Fig. 2).
If the elevated horizontal stress exceeds the yield strength of
the rocks, the margins of the extended terrane could collapse inward
Figure 11. Dynamic model for the formation of extensionparallel folds as expressed in cross-section views perpendicular to
the extension direction. (A) Before denudation the horizontal
stresses on the boundary and within the block are equal because the
integrated horizontal force operates over the same crustal thickness: Dhint and Dhext. (B) After denudation, force balance requires
horizontal stress to increase in the thinned portion of the crust: the
integrated horizontal force remains constant but is distributed over
a shorter distance in the denuded portion of the crust (Dhint <
Dhext). Additionally, vertical stress decreases at a given horizon in
the footwall in response to denudation. Lower boundary of extended
upper crust could be low-viscosity compensating material like that
proposed by Block and Royden (1990). (C) The crust may respond
by shortening in the denuded region and extending along the lateral
margins.
Geological Society of America Bulletin, December 1995
1479
FLETCHER ET AL.
crust exists in the wide variety of extended continental and oceanic
crust remains to be demonstrated. Additionally, these mechanisms
should produce extension above the antiformally deflected crust
that is not observed in the central Mojave metamorphic core
complex.
Some workers have interpreted extension-parallel folds/undulations to represent original corrugations in the fault (Davis and
Hardy, 1981; Spencer, 1985; John, 1987; Davis and Lister, 1988)
which may form by linking of separate straight fault segments during
the early stages of fault propagation. Concordance between the brittle fault and the lower-plate mylonites would require either that the
mylonites were ductilely molded to the base of the rigid hangingwall block (Spencer and Reynolds, 1991) or that corrugations of the
brittle fault project downward into similar corrugations in the ductile regime. However, neither of these mechanisms is likely to produce widespread constrictional strain; molding the footwall mylonite to a nonplanar hanging-wall block should actually result in
flattening strains.
Extension-Perpendicular Folds. The broad antiformal arch of
fault zones in nearly all core complexes consists of a limb that dips
in the same direction as the active shear zone and one tilted through
horizontal to show apparent reverse-sense displacement. Following
Reynolds and Lister (1990), we refer to these as the fore- and backdipping limbs, respectively. In the central Mojave metamorphic core
complex, the antiformal arch is best preserved in the Mitchel Range
where it is asymmetric: the fore-dipping limb dips 108–208 and the
back-dipping limb dips 408–508. Although we cannot rule out local
contraction across the Harper Lake fault (e.g., Bartley et al., 1990b)
as the cause of this asymmetry, other Cordilleran core complexes
show a similar geometry (e.g., Reynolds and Lister, 1990; Lister and
Baldwin, 1993).
The only mesoscopic fabrics that could be related to this longwavelength northwest-trending fold are joints and the composite
kink-band and tension-gash cleavage. Both fabrics formed postmylonitically, predominantly accomplish northeast-side-up subvertical
shear and/or layer-parallel extension, and are primarily found in
previously mylonitized rocks. North-east-side-up shear is mainly accomplished by monoclinal forms of the composite cleavage (Fig. 4B)
that is preferentially developed on the back-dipping limb. Joints are
common on both the fore- and back-dipping limbs of the antiform.
The most popular models for extension-perpendicular folds invoke heterogeneous vertical stress that arises from buoyant loads
generated either by tectonic denudation or by magmatic inflation.
Uplift above a symmetric low-density pluton should produce an
asymmetrically warped fault zone with a steeper fore-dipping limb,
because the fault initially dipped in this direction. This is the opposite of the observed fault-zone geometry in the central Mojave
other metamorphic core complexes (e.g., Reynolds and Lister,
1990). Also, tectonic removal of crust should generate much larger
vertical loads than density differences between granitic plutons and
country rock because the density contrast between rock and air is
greater than the density contrast between any two crustal rock types.
Therefore, although low-density plutons may induce significant vertical loads in core complexes, we consider tectonic denudation to be
the dominant source of vertical loads in the central Mojave metamorphic core complex.
Recent models of isostatic rebound of tectonically denuded
crust predict a center of uplift that migrates through the footwall as
1480
it follows the trailing edge of the displaced hanging wall (Buck, 1988;
Wernicke and Axen, 1988). These models predict deflections of the
crust that vary in time and space and thus at least permit the possibility for folds with steeper back-dipping limbs to occur. Differential tectonic unroofing might also explain the fold asymmetry
(Spencer, 1984).
Determining the nature of crustal deflections that occur in response to the migrating center of uplift or ‘‘rolling hinge’’ has been
the focus of many recent studies (e.g., Bartley et al., 1990a; Axen and
Wernicke, 1991; Selverstone et al., 1993; Manning and Bartley,
1994). Based upon the preliminary results from the Mitchel domain,
Bartley et al. (1990a) interpreted the postmylonitic deformational
fabrics to reflect isostatic rebound by flexural failure, but Axen and
Wernicke (1991) pointed out that the data were equally consistent
with rebound by subvertical noncoaxial shear. The data presented in
this paper shed more light on the controversy.
The predominance of the monoclinal composite fabric on the
back-dipping limb suggests that the rotation of this portion of the
shear zone through horizontal included a significant component of
northeast-side-up shear (Fig. 12). Although the composite fabric is
more penetrative, we liken it to the antithetic shear zones that Reynolds and Lister (1990) interpreted to have caused back-rotation of
the shear zone in the Harcuvar Mountains, South Mountains, and
Santa Catalina Mountains in southern Arizona. The origin of joints
and mesoscopic normal faults is more uncertain. These could have
formed in response to extension around the outer arc of the antiformal flexure at the top of a fault ramp (Fig. 12), as similar structures were interpreted in the Raft River Mountains (Manning and
Bartley, 1994). Alternatively, they also could have accommodated
layer-parallel extension at the synformal hinge at the bottom of a
fault ramp (cf. Wernicke and Axen, 1988, Fig. 4c). Each interpretation is consistent with the fanning of joint orientations around the
fold. Other important aspects of rolling-hinge models, such as
whether the antiform migrated through the footwall or if the fault
had a steep orientation while active, cannot be answered with this
data set.
Figure 12. Schematic cross sections showing kinematics of the
rolling hinge: subvertical shear on back-dipping limb and flexural
failure around hinge of antiform.
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
Role of Magmatism in Extensional Deformation
The common temporal and spatial coincidence of igneous activity and deformation in many core complexes has led workers to
infer genetic relationships between the two phenomena. On the
lithospheric scale, the most commonly inferred link involves thermal
weakening of gravitationally unstable crust either through thermal
relaxation following contraction (Glazner and Bartley, 1985), elevated mantle heat flow (Sonder et al., 1987; Wernicke et al., 1987;
Axen, 1993), or the emplacement of mafic magma in the lower crust
(Gans et al., 1989). The initial thermal pulse triggers extensional
collapse, which leads to smaller-scale sympathetic relationships between magmatism and extensional deformation. Some of the ways
that magmatism can enhance extensional deformation on a smaller
scale include (1) localized thermal weakening on the margins of
plutons and dikes, (2) reorientation of stress trajectories and development of locally elevated deviatoric stress around margins of
magma bodies (e.g., Pollard and Seagall, 1987; Parsons and Thompson, 1993), and (3) magmatic hydrofracture or brittle failure in response to elevated pore pressure associated with magma. Some of
the ways that extensional deformation can induce the generation
and migration of magma include (1) rapid isothermal decompression by tectonic denudation and (2) creation of dilatant openings
formed as a result of rock failure and strain incompatibilities.
Lister and Baldwin (1993) proposed that local thermal anomalies associated with midcrustal intrusions (plutons, dikes, and dike
swarms) trigger core-complex-style extension. They cited examples
in the southern Basin and Range of ductile strain gradients around
intrusions and suggested that cooling-age patterns record transient
geotherms associated with midcrustal igneous intrusion. These authors specifically predicted the presence of syn- or prekinematic
igneous rocks in the immediate vicinity of ductilely deformed footwall rocks and suggested that drilling or thermochronology could be
used to identify them and test their hypothesis. However, we suggest
that other field criteria may also shed light on this issue.
On the scale of the Mojave Desert as a whole, outcrops of
Miocene granite coincide with areas of brittle-ductile extensional
detachment. However, variations in the intensity of ductile deformation do not correlate with the extent of granite exposures and, in
the Mitchel domain where pluton/wall-rock relationships are best
exposed, most of the ductile deformation preceded emplacement of
the pluton.
The central Mojave metamorphic core complex contains rare
examples of strain localization around the margins of dikes (e.g., in
the Buttes). However, more typically igneous rocks display no preferential development of ductile strain. The vast majority of dikes
were intruded after mylonitization (e.g., Hinkley Hills) or into wall
rock that records no ductile strain (e.g., at Fremont Peak and Lead
Mountain) (Fig. 1). The undeformed dikes generally are oriented
parallel to the joint fabric, which may have aided and controlled
their emplacement. We suggest below that postmylonitic dikes in
the Hinkley domain may have been emplaced in rocks located near
the brittle-ductile transition at that time. However, most of the ductile shearing occurred in the structurally lower Mitchel domain, despite the fact that it contains significantly fewer dikes. Finally, because most of the dikes in the central Mojave metamorphic core
complex were emplaced after or during mylonitization, one could
perhaps argue more successfully that ductile extension triggered the
migration of magma into the middle and upper crust rather than the
reverse.
In summary, the general coincidence of ductile extension and
plutonism in the central Mojave Desert are broadly consistent with
potential genetic relationships on the lithospheric scale. However,
on the scale of the domains in this study, field relations do not
support any single cause-and-effect relationship between ductile extension and magmatic intrusions.
Thickness of the Brittle-Ductile Transition
The contrast in relative timing of dike emplacement in Hinkley
Hills and Mitchel Range may provide a snapshot of variations in
deformational style in the down-dip direction of the central Mojave
metamorphic core complex fault zone. In the Hinkley Hills, dikes
crosscut semiductile mesoscopic fabrics in the footwall but are
themselves crosscut by the brittle detachment. However, structurally
down-dip in the Mitchel Range, dikes are crosscut by and, in most
cases, fully transposed into parallelism with the footwall mylonitic
fabric. These relations suggest dikes in the Hinkley Hills were emplaced after the footwall rocks had passed through the brittle-ductile
transition but well before ductile deformation had ceased in the
footwall rocks of the Mitchel Range. We infer that postemplacement offset of the dikes occurred almost entirely across the brittle
detachment in the Hinkley Hills, whereas, in the Mitchel Range, this
offset predominantly occurred across the ductile shear zone
(Fig. 13).
After displacement across the younger Harper Lake Fault is
restored, ;2 km in the down-dip dimension separates brittlely deformed dikes in the Hinkley Hills from ductilely deformed dikes in
the Mitchel Range. If the dikes were emplaced synchronously in
both areas, we would interpret the intervening 2 km of unexposed
ground to contain the former brittle-ductile transition. The maximum thickness of the transition depends on the active dip of the
fault zone and the amount of ductile stretching that occurred between the Hinkley Hills and Mitchel Range before rocks exposed in
the Mitchel Range were transported passively to the surface in the
footwall of the brittle detachment (Fig. 13). Assuming active fault
inclinations of 158– 458 and ductile stretches of 1.5–5.0, the maximum vertical thickness of the brittle-ductile transition is 100 –950 m
(Fletcher, 1994). This estimation is almost an order of magnitude
thinner than thicknesses predicted by monomineralic deformation
experiments (e.g., Hirth and Tullis, 1994) and flow laws of phases
that make up the strongly heterogeneous continental crust like that
in the central Mojave (e.g., Kirby and Kronenberg, 1987). Future
studies in the central Mojave metamorphic core complex will more
completely characterize the age of the suite of dikes in both areas
and explain the discrepancy between measured and predicted transition thicknesses.
CONCLUSIONS
(1) Synmylonitic folds exhibit a wide range of styles and generally have axes parallel to the extension direction of the shear zone.
We infer that they formed mainly by two main processes: (a) amplification of upright open folds that initially contained axes nearly
parallel to the transport direction and (b) transposition of a preexisting steep northeast-striking gneissic fabric into parallelism with
Geological Society of America Bulletin, December 1995
1481
FLETCHER ET AL.
after mylonitization and records layer-parallel extension and northeast-side-up subvertical shear that may reflect isostatic rebound of
the footwall.
(3) The regional distribution of magmatism and large-magnitude brittle-ductile extension broadly coincide. However, on the
scale of individual mountain ranges, the relative timing of magmatism and ductile deformation varies throughout the central Mojave
metamorphic core complex. Additionally, shear zone thickness and
degree of mylonitization show no correlation with proximity to or
volume of extension-related intrusions. Therefore we are skeptical
of models that suggest that midcrustal magmatism triggers extension
or vice versa. Instead, we prefer the hypothesis that large-magnitude
extension and magmatism both are manifestations of common larger-scale processes in the lithosphere.
(4) A preserved crustal section containing the brittle-ductile
transition may be present in the central Mojave metamorphic core
complex. Approximately 2 km of dip-parallel distance separates a
portion of the footwall where dikes are strongly overprinted by mylonitization from another portion where lithologically identical
dikes postdate mylonitization but are crosscut by the brittle detachment. If the dikes were emplaced over the same time interval, those
in the Mitchel Range would have been emplaced below the brittleductile transition and those in the Hinkley Hills would have been
emplaced above the transition.
ACKNOWLEDGMENTS
This study was partially funded by National Science Foundation
grants EAR8816944 and EAR8916838. John Bendixen and Rob
Fillmore assisted in various aspects of the field work. Stereograms
were generated using Stereonet 4.5a by Richard Allmendinger.
Thoughtful reviews by Keith Howard, Steve Reynolds, and Jon
Spencer added considerably to this paper.
REFERENCES CITED
Figure 13. Model for emplacement of dikes in the Hinkley Hills
and Mitchel Range showing potential constraints on the vertical
thickness of the early Miocene brittle-ductile transition in the central Mojave. The thickness of the transition depends on the active
dip of the fault zone (f) and the stretch (lf /li) that occurred between
the Hinkley Hills (HH) and Mitchel Range (MR) after the dikes
were emplaced.
the shallow mylonitic foliation. The first process would produce
upright open folds, moderately inclined asymmetric folds with interlimb angles .908, and sheath folds, whereas the second would
largely produce recumbent folds.
(2) The two fold sets that define the basin-and-dome geometry
of the fault zone are likely to result from mechanically independent
processes. The short-wavelength fold set parallel to the extension
direction formed during active displacement across the fault zone
and records true crustal shortening that may have resulted from
inward collapse of the margins of the extended terrane. The longwavelength fold set perpendicular to the extension direction formed
1482
Andersen, T. B., and Jamveit, B., 1990, Uplift of deep crust during orogenic extensional collapse; a
model based on field studies in the Sogn-Sunnfjord region western Norway: Tectonics, v. 9,
p. 1097–1111.
Axen, G. J., 1993, Ramp-flat detachment faulting and low-angle normal reactivation of the Tule
Springs thrust, southern Nevada: Geological Society of America Bulletin, v. 105, p. 1076–1090.
Axen, G. J., and Wernicke, B. P., 1991, Comment on ‘‘Tertiary extension and contraction of lower
plate rocks in the Central Mojave metamorphic core complex, southern California’’ by John M.
Bartley, John M. Fletcher and Allen F. Glazner: Tectonics, v. 10, p. 1084–1086.
Baldwin, S. L., Lister, G. S., Hill, E. J., Foster, D. A., and McDougall, I., 1993, Thermochronologic
constraints on the tectonic evolution of active metamorphic core complexes, D’Entrecasteaux
Islands, Papua New Guinea: Tectonics, v. 12, p. 611– 628.
Bartley, J. M., and Glazner, A. F., 1991, En echelon Miocene rifting in the southwestern United States
and a model for vertical-axis rotation in continental extension: Geology, v. 19, p. 1165–1168.
Bartley, J. M., Fletcher, J. M., and Glazner, A. F., 1990a, Tertiary extension and contraction of lower
plate rocks in the central Mojave metamorphic core complex, southern California: Tectonics,
v. 9, p. 521–534.
Bartley, J. M., Glazner, A. F., and Schermer, E. R., 1990b, North-south contraction of the Mojave
block and strike-slip tectonics in southern California: Science, v. 248, p. 1398–1401.
Block, L., and Royden, L., 1990, Core-complex geometries and regional-scale flow in the lower crust:
Tectonics, v. 9, p. 557–567.
Boettcher, S. S., and Walker, J. D., 1993, Geologic evolution of Iron Mountain, central Mojave desert,
California: Tectonics, v. 12, p. 372–386.
Buck, R., 1988, Flexural rotation of normal faults: Tectonics, v. 7, p. 959–973.
Burchfiel, B. C., and Royden, L. H., 1985, North-south extension within the convergent Himalayan
region: Geology, v. 13, p. 679– 682.
Burg, J. P., Brunel, M., Gapais, D., Chen, G. M., and Liu, G. H., 1984, Deformation of leucogranites
of the crystalline main central thrust sheet in southern Tibet (China): Journal of Structural
Geology, v. 6, p. 535–542.
Cannat, M., 1987, Plastic deformation at an oceanic spreading ridge: A microstructural study of Site
735 gabbros (southwest Indian Ocean), in Stewart, S. K., ed., Fracture zone drilling on the
Southwest Indian Ridge: Scientific Results Volume 118, Proceedings of the Ocean Drilling
Program, p. 399– 408.
Cannat, M., Mevel, C., and Stakes, D., 1987, Normal ductile shear zones at an oceanic spreading ridge:
Tectonic evolution of Site 735 gabbros (southwest Indian Ocean), in Stewart, S. K., ed., Fracture
zone drilling on the Southwest Indian Ridge: Scientific Results Volume 118, Proceedings of the
Ocean Drilling Program, p. 415– 430.
Compton, R. R., 1975, Geologic map of the Park Valley quadrangle, Box Elder County, Utah
Geological Society of America Bulletin, December 1995
LARGE-MAGNITUDE CONTINENTAL EXTENSION
and Cassio County Idaho: U.S. Geological Survey Miscellaneous Investigations 1-873,
scale 1:31 680.
Davies, H. L., and Warren, R. G., 1988, Origin of eclogite-bearing, domed, layered metamorphic
complexes (‘‘core complexes’’) in the D’Entrecasteaux Islands, Papua New Guinea: Tectonics,
v. 7, p. 1–21.
Davis, G. H., and Hardy, J. J., 1981, Eagle pass detachment, southeastern Arizona: A product of
Miocene (?) normal faulting in the southern Basin and Range province: Geological Society of
America Bulletin, v. 92, p. 749–762.
Davis, G. A., and Lister, G. S., 1988, Detachment faulting in continental extension; Perspectives from
the southwestern U.S. Cordillera, in Clark, S. P., Burchfiel, B. C., and Suppe, J., eds., Processes
in continental lithospheric deformation (John Rodgers Symposium Volume): Geological Society of America Special Paper 218, p. 133–159.
Dick, H. J. B., Meyer, P. S., Bloomer, S., Kirby, S., Stakes, D., and Mawer, C., 1987a, Lithostratigraphic
evolution of an in situ section of oceanic layer 3, in Stewart, S. K., ed., Fracture zone drilling
on the Southwest Indian Ridge: Scientific Results Volume 118, Proceedings of the Ocean
Drilling Program, p. 439–540.
Dokka, R. K., 1989, The Mojave Extensional Belt of southern California: Tectonics, v. 8, p. 363–390.
Dokka, R. K., McCurry, M., Woodburne, M. O., Frost, E. G., and Okaya, D. A., 1988, A field guide
to the Cenozoic structure of the central Mojave Desert, in Weide, D. L., and Faber, M. L., eds.,
This extended land—Geological journeys in the southern Basin and Range: Geological Society
of America Guidebook, Cordilleran Section Meeting, Las Vegas, University of Nevada,
p. 21– 49.
Dokka, R. K., and ten others, 1991, Aspects of the Mesozoic and Cenozoic geologic evolution of the
Mojave Desert, in Walawender, M. J., and Hanan, B. B., eds., Geological excursions in southern
California and Mexico: Guidebook, Annual Meeting, Geological Society of America: San Diego, San Diego State University, p. 1– 43.
Duebendorfer, E. M., and Simpson, D. A., 1994, Kinematics and timing of Tertiary extension in the
Lake Mead region, Nevada: Geological Society of America Bulletin, v. 106, p. 1057–1073.
Fillmore, R. P., and Walker, J. D., in press, Evolution of a supradetachment extensional basin: The
early Miocene Pickhandle basin, central Mojave Desert, California, in Beratan, K., ed., Reconstructing the structural history of Basin and Range extension using sedimentology and
stratigraphy: Geological Society of America Special Paper 303.
Fletcher, J. M., 1994, Geodynamics of large-magnitude extension: A field-based study of the central
Mojave metamorphic core complex [Ph.D. dissert.]: Salt Lake City, University of Utah, 109 p.
Fletcher, J. M., and Bartley, J. M., 1994, Constrictional strain in a noncoaxial shear zone; implications
for fold and rock fabric development, central Mojave Metamorphic Core complex. California:
Journal of Structural Geology, v. 16, no. 4, p. 555–570.
Gans, P. B., Mahood, G. A., and Schermer, E. R., 1989, Synextensional magmatism in the Basin and
Range province; a case study from the eastern Great Basin: Geological Society of America
Special Paper 233, 60 p.
Glazner, A. F., and Bartley, J. M., 1985, Evolution of lithospheric strength after thrusting: Geology,
v. 13, p. 42– 45.
Glazner, A. F., Walker, J. D., and Bartley, J. M., 1989, Magnitude and significance of Miocene crustal
extension in the central Mojave Desert, California: Geology, v. 17, p. 50–53.
Glazner, A. F., and nine others, 1994, Reconstruction of the Mojave Block, in McGill, S. F., and Ross,
T. M., eds., Geologic investigations of an active margin, Geological Society of America Cordilleran Section Guidebook: Redlands, California, San Bernardino County Museum Association, p. 3–30.
Harper, G. D., 1985, Tectonics of slow-spreading mid-ocean ridges and consequences of variable
depth to the brittle-ductile transition: Tectonics, v. 4, p. 395– 409.
Hill, E. J., Baldwin, S. L., and Lister, G. S., 1992, Unroofing of active metamorphic core complexes
in the D’Entrecasteau Islands, Papua New Guinea: Geology, v. 20, p. 907–910.
Hirth, G., and Tullis, J., 1994, The brittle plastic transition in experimentally deformed quartz aggregates: Journal of Geophysical Research, v. 99, no. B6, p. 11731–11747.
John, B. E., 1987, Geometry and evolution of a mid-crustal extensional fault system: Chemihuevi
Mountains, southeastern California, in Coward, M. P., Dewey, J. F., and Hancock, P. L., eds.,
Continental extensional tectonics: Geological Society [London] Special Publication 28,
p. 313–335.
Kirby, S. I., and Kronenberg, A. K., 1987, Rheology of the lithosphere: Selected topics: Journal of
Geophysical Research, v. 25, no. 6, p. 1219–1244.
Kiser, N. L., 1981, Stratigraphy structure and metamorphism in the Hinkley Hills, Barstow, California
[M.S. thesis]: Palo Alto, California, Stanford University, 95 p.
Lee, J., and Lister, G. S., 1992, Late Miocene ductile extension and detachment faulting, Mykonos,
Greece: Geology, v. 20, p. 121–124.
Lister, G. S., and Baldwin, S. L., 1993, Plutonism and the origin of metamorphic core complexes:
Geology, v. 21, p. 607– 610.
Mancktelow, N. S., and Pavlis, T. L., 1994, Fold-fault relationships in low-angle detachment fault
systems: Tectonics, v. 13, p. 668– 685.
Manning, A. H., and Bartley, J. M., 1994, Postmylonitic deformation in the Raft River metamorphic
core complex, northwestern Utah: Evidence of a rolling hinge: Tectonics, v. 13, p. 596– 612.
Martin, M. W., Glazner, A. F., Walker, J. D., and Schermer, E. R., 1993, Evidence for right-lateral
transfer faulting accommodating en echelon Miocene extension, Mojave Desert, California:
Geology, v. 21, p. 355–358.
Mpodozis, C., and Allmendinger, R. W., 1993, Extensional tectonics, Cretaceous Andes, northern
Chile (278S): Geological Society of America Bulletin, v. 105, p. 1462–1477.
Norton, M. G., 1986, Late Caledonian extension in western Norway: A response to extreme crustal
thickening: Tectonics, v. 5, p. 195–204.
Parsons, T., and Thompson, G. A., 1993, Does magmatism influence low-angle normal faulting?:
Geology, v. 21, p. 247–250.
Pollard, D. D., and Seagall, P., 1987, Theoretical displacements and stresses near fractures in rock:
With applications to faults, joints, veins, dikes, and solution surfaces, in Atkinson, B. K., ed.,
Fracture mechanics of rock: London, Academic Press, p. 277–350.
Ramsay, J. G., and Huber, M. I., 1987, Techniques of modern structural geology, Volume 2: Folds and
fractures: London, Academic Press, 700 p.
Rehrig, W. A., and Reynolds, S. J., 1980, Geologic and geochronologic reconnaissance of a northwesttrending zone of metamorphic core complexes in southern and western Arizona, in Crittenden,
M. D., Coney, P. J., and Davis, G. H., eds., Cordilleran metamorphic core complexes: Geological
Society of America Memoir 153, p. 131–158.
Reynolds, S. J., and Lister, G. S., 1990, Folding of mylonitic zones in Cordilleran metamorphic core
complexes: Evidence from near the mylonitic front: Geology, v. 18, p. 216–219.
Selverstone, J., Axen, G., and Bartley, J. M., 1993, P-T conditions of successive fracturing events during
unroofing of an extensional mylonite zone: Constraints from oriented fluid-inclusion planes:
Geological Society of America Abstracts with Programs, v. 25, no. 6, p. A 423.
Sonder, L. J., England, P. C., Wernicke, B. P., and Christiansen, R. L., 1987, A physical model for
extension of western North America, in Coward, M. P., Dewey, J. F., and Hancock, P. L., eds.,
Continental extensional tectonics: Geological Society [London] Special Publication No. 28,
p. 187–202.
Spencer, J. E., 1982, Origin of folds of Tertiary low-angle fault surfaces, southeastern California and
western Arizona, in Frost, E. G., and Martin, D. L., eds., Mesozoic-Cenozoic tectonic evolution
of the Colorado River region, California, Arizona, and Nevada (Anderson-Hamilton Volume):
San Diego, California, Cordilleran Publishers, p. 123–134.
Spencer, J. E., 1984, Role of denudation in warping and uplift of low-angle normal faults: Geology,
v. 12, p. 95–98.
Spencer, J. E., 1985, Miocene low-angle normal faulting and dike formation, Homer Mountains and
surrounding areas, southeastern California: Geological Society of America Bulletin, v. 96,
p. 1140–1155.
Spencer, J. E., and Reynolds, S. J., 1991, Tectonics of mid-Tertiary extension along a transect through
west-central Arizona: Tectonics, v. 10, p. 1204–1221.
Varga, R. J., and Moores, E. M., 1985, Spreading structure of the Troodos ophiolite, Cyprus: Geology,
v. 13, p. 846– 850.
Walker, J. D., Bartley, J. M., and Glazner, A. F., 1990, Large-magnitude Miocene extension in the
central Mojave Desert: Implications for Paleozoic to Tertiary paleogeography and tectonics:
Journal of Geophysical Research, v. 95, p. 557–569.
Walker, J. D., Fletcher, J. M., Fillmore, R. P., Martin, M. W., Taylor, W. J., Glazner, A. F., and Bartley,
J. M., 1995, Connection between igneous activity and extension in the central Mojave metamorphic core complex, California: Journal of Geophysical Research, v. 100, no. B7,
p. 10477–10494.
Wernicke, B., 1990, The fluid crustal layer and its implications for continental dynamics, in Salisbury,
M. H., and Fountain, D. M., eds., Exposed cross sections of the continental crust: Dordrecht,
Netherlands, Kluwer Academic Publishers, p. 509–544.
Wernicke, B. P., 1992, Plate 8. Areas of strong upper crustal extension, in Burchfiel, B. C., Lipman,
P. W., and Zoback, M. L., eds., The Cordilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-3, p. 724.
Wernicke, B., and Axen, G. J., 1988, On the role of isostasy in the evolution of normal fault systems:
Geology, v. 16, p. 848– 851.
Wernicke, B. P., Christiansen, R. L., England, P. C., and Sonder, L. J., 1987, Tectonomagmatic
evolution of Cenozoic extension in the North American Cordillera, in M. P., C., Dewy, J. F., and
Hancock, P. L., eds., Continental extensional tectonics: Geological Society [London] Special
Publication No. 28, p. 203–221.
Yin, A., 1991, Mechanisms for the formation of domal and basinal detachment faults: A threedimensional analysis: Journal of Geophysical Research, v. 96, p. 14577–14594.
Yin, A., and Dunn, J. F., 1992, Structural and stratigraphic development of the Whipple-Chemehuevi
detachment fault system, southeastern California: Implications for the geometrical evolution of
domal and basinal low-angle normal faults: Geological Society of America Bulletin, v. 104,
p. 659– 674.
MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 28, 1994
REVISED MANUSCRIPT RECEIVED MAY 5, 1995
MANUSCRIPT ACCEPTED MAY 30, 1995
Printed in U.S.A.
Geological Society of America Bulletin, December 1995
1483