High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.
In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.
Metamorphic core complexes are exhumed sections of the ductile middle crust brought to the surface during horizontal crustal extension and vertical thinning. They were first recognized and described in the North American Cordillera (Coney, 1980), where they form a discontinuous NW–SE-trending belt extending from Canada to Mexico in the hinterland of the Sevier fold and thrust belt (Fig. 1). Characteristically, these core complexes exhibit a low-angle domiform detachment surface that separates unmetamorphosed rocks cut by brittle normal faults in the hanging wall from metamorphic rocks with predominantly ductile fabrics in the footwall (for comprehensive core complex reviews, see Platt et al., 2015; Whitney et al., 2013).
Simplified geological maps and cross sections of the three
study areas. In each case, the mylonitic lineation is shown schematically.
Ductile strain in the footwalls may extend several hundreds of meters to a few kilometers below the detachment, increasing in intensity upward to a zone of mylonite directly beneath it. Mylonitic rocks in these zones of high strain typically show evidence of crystal-plastic deformation and dynamic recrystallization in quartz, and contain porphyroclasts of feldspar and mica fish. The strain is non-coaxial and generally has a shear sense consistent with the brittle detachment above.
The top of the mylonite zone is commonly overprinted by a narrow zone of cataclasite, directly beneath the discrete brittle slip surface of the detachment (Coney, 1980; Davis and Lister, 1988; Miller et al., 1983). However, the precise relationship between the mylonites and the detachment is not always clear. Some mylonites may reflect ductile deformation related to exhumation along the detachment itself, whereas others are likely to have formed at greater depth and been subsequently “captured” by the detachment (Davis and Lister, 1988). During exhumation, one type may be superposed on the other, making interpretation difficult. Overprinting of the mylonites by later cataclastic deformation suggests that they pre-date brittle displacement on the detachment and, in at least one example (the Whipple Mountains, California), mylonitic gneisses and the brittle detachment separate in the up-dip direction, indicating that they are not directly related (Davis, 1988).
Semicontinuous north–south belt of Cordilleran metamorphic core complexes running from Canada to Mexico in the hinterland of the Sevier thrust belt. The three complexes focused on in this study are shown in black: 1, Whipple Mountains; 2, northern Snake Range; 3, Ruby Mountains–East Humboldt Range. Redrawn from Wong and Gans (2008) and Cooper et al. (2010b), modified from Coney (1980) and Wernicke (1992). Arrows indicate hanging wall transport directions after Wust (1986).
Rapid exhumation of most core complex footwalls means that early formed microstructures are not destroyed by re-equilibration, and they can thus preserve a snapshot through the middle and upper crust prior to exhumation. In particular, they can record changes in crustal rheology with depth. For example, in the northern Snake Range metamorphic core complex in Nevada, Cooper et al. (2010b) distinguished two preserved mid-crustal rheological transitions in the exhumed mylonitic footwall rocks: (1) a brittle–ductile transition (BDT), marking the downward progression from discrete brittle faulting to continuous, but localized, ductile shear, and (2) a localized–distributed transition (LDT), a deeper, temperature-dependent transition marking the switch from localized ductile shear to distributed ductile flow. The authors concluded that this preserved sequence of deformation reflects moderate- to high-angle brittle normal faulting in the upper crust that persisted as low-angle ductile shear zones below the BDT in the middle crust, soling into the LDT at greater depths. The bulk of the high-strain rocks preserved in the footwall are, therefore, a pre-existing feature of the ductile middle crust that was subsequently captured and exhumed by a brittle detachment (the northern Snake Range décollement, NSRD). In this interpretation, the brittle hanging wall to the NSRD represents a series of upper crustal normal faults, whereas the ductile footwall represents exhumed middle crustal ductile deformation, much of which is not directly related to the detachment.
In this paper we address two questions concerning mid-crustal rheology raised by these fundamental observations. (1) How do the mylonites and other deformed rocks in a core complex footwall relate to the detachment that exhumes them? (2) What do these relationships tell us about the rheological structure of the middle and lower crust? In an attempt to answer these questions, we present a detailed investigation of crustal rheology in the footwalls of the Whipple Mountains, the northern Snake Range, and the Ruby Mountains–East Humboldt Range in northeast Nevada (Fig. 2). Using field observations, microstructural analyses, and thermobarometric data, we (i) document the exhumation of footwall rocks from the middle crust to the surface in each core complex as they pass through different rheological transitions; (ii) document the exhumed LDT in the Whipple Mountains and show why it is not easily identified in the northern Snake Range, and why it has not been exhumed in the Ruby Mountains–East Humboldt Range; (iii) present a prediction of, and a mechanical explanation for, the geometry of the detachments in these three core complexes and many like them; and (iv) show that the detachments in all three core complexes formed during the Miocene and post-date early phases of extension and exhumation in the exhumed mid-crustal metamorphic rocks.
A schematic representation of the spatial relationships of
different rheological styles within crust cut by an extensional detachment,
prior to displacement on the fault. Geometrical features of the detachment
are shown in italics and rheological features in regular type. See text for
full discussion and references. BDT: brittle–ductile transition; LDT: localized–distributed transition. Note that in metamorphic core complexes
the rheological features are superposed on one another during exhumation,
and some elements may be excised. Insets show outcrop-scale features
observed in the Whipple Mountains core complex. Mini-detachments are
brittle, brittle–ductile, or narrow ductile shear zones, with displacements
of tens to hundreds of meters that splay off the main detachment (Axen
and Selverstone, 1994; Luther et al., 2013; Selverstone et al., 2012). They
may form at any depth down to the LDT. Vertical scale is drawn for an
average geothermal gradient of 25
Core complex footwalls can offer a unique glimpse into the middle to
lower crust, allowing us to define a sequence of deformational styles,
rheological behaviors, and structures as a function of depth
(e.g., Cooper et al., 2010b). Because their formation
results in the rapid tectonic exhumation of rock from the ductile middle
crust up to shallow crustal levels, they provide well-constrained natural
rock-mechanical experiments conducted under geological conditions of
strain-rate and temperature (e.g., Behr and Platt, 2011). As rocks
are exhumed and cooled, they experience progressive strain localization with
higher overall levels of stress in the younger, lower-temperature parts of
the sequence. During extension, each step in this progressive localization
is collapsed onto the last, resulting in the superposition of structures
from different crustal levels. The resulting distinctive sequence of
structures and microstructures inform us of the mechanics of deformation as
the rocks are exhumed. These are summarized in detail by Platt et al. (2015) and shown schematically in Fig. 3. To illustrate the concepts
here we will imagine a rock as it is exhumed towards the surface from a
depth of
Our rock starts out at a depth of
As the rock is exhumed towards the surface and cools, it crosses a
sub-horizontal transition into a mid-crustal zone of localized deformation.
Following the study by Cooper et al. (2010b), we refer to this
transition as the localized–distributed transition (LDT). The LDT can be
defined as a boundary below which there is no evidence for strain
localization. The transition itself is likely to be marked by a zone of
relatively high strain as it separates distributed deformation below from
narrow ductile shear zones bounding kilometer-scale undeformed crustal blocks
above. Large-scale upper crustal faults are also likely to sole into the LDT
and transfer displacement onto it. Our observations suggest that the LDT
commonly occurs at a depth of 18–20 km, where the temperature is
Above the LDT, ductile deformation is strongly localized into one or more
shear zones with a cumulative width of a few tens to
Close to the brittle–ductile transition (BDT), extreme grain-size reduction
by dynamic recrystallization and mixing of phases (e.g., quartz, feldspar,
chlorite, and micas) results in the formation of fine-grained
ultramylonites. Dynamically recrystallized quartz shows the characteristic
Regime 1 microstructures of Hirth and Tullis (1992), and grain-size
reduction may cause a switch to dislocation-accommodated grain-boundary
sliding (Behr and Platt, 2013; Behrmann, 1985). Shear stresses
around 60–100 MPa mean that the detachment here is likely to have dipped
> 30
The final stage in our rock's journey to the surface is accommodated by
motion along the brittle detachment. Near the base of the brittle zone,
rocks are typically enriched in hydrothermal minerals such as feldspar,
quartz, mica, and chlorite, resulting in a breccia or microbreccia texture,
high coefficients of friction, and faults dipping > 30
Our discussion up to this point is based on observational evidence for
changes in the width and character of a detachment zone as a function of
depth. These changes suggest progressively decreasing localization (and
hence increasing shear zone width) with increasing depth below the BDT, and
little to no strain localization below the LDT. In order to quantify this
behavior in terms of the rheology of the mylonitic rocks, we carried out
some simple strain-rate calculations. Figure 4a shows the relationship
between strain rate and depth calculated using the stress–depth profile
determined by Behr and Platt (2011) for the Whipple Mountains core
complex, together with several flow laws for both quartzite (Q1–3) and
granite (G1–3). Figure 4b shows how the width of a ductile shear zone (or
cumulative width of several shear zones) varies as a function of depth using
the calculated strain rates and assuming a slip rate of 5 mm yr
Our calculations are strongly sensitive to a number of assumptions (see
caption to Fig. 4), so the results cannot be considered representative of
all such detachment-related mylonite zones. Assumptions include (i) the
geothermal gradient; (ii) the stress distribution with depth; and (iii), for
granitic mylonites, that the quartz forms an interconnected weak layer
microstructure (Handy, 1994), such that its mechanical behavior
corresponds to the Reuss (constant stress) condition for a polyphase
aggregate. The most critical assumption, however, is that (iv) the crust is
relatively dry: we assume a constant water fugacity of 28 MPa with depth.
This corresponds to water saturation at the depth of the BDT (12 km,
300
To illustrate how these concepts explain the relationships seen in metamorphic core complexes, where the various rheological levels have been exhumed and juxtaposed by crustal thinning, we describe three core complex footwalls from the North American Cordillera with which we are familiar. Together, they highlight the rheological transitions described above, although they show differences in detail due to variations in pre-extensional tectonic and geological setting and the amount of extension.
The Whipple Mountains metamorphic core complex (WMCC) in southeast California is one of the classic Cordilleran core complexes (Fig. 1). First recognized as such and described by Davis et al. (1980), it is representative of the style of core complex developed in the Colorado River extensional corridor (Davis et al., 1986) and adjacent areas in Arizona (Spencer and Reynolds, 1991). Together with the Buckskin, Rawhide, and possibly other nearby core complexes it may be part of a continuous larger extensional structure (Davis and Lister, 1988). The WMCC shows the distinctive domiform geometry of many core complexes, with a continuous, clearly defined detachment that dips gently away from the metamorphic core in all directions (Fig. 2a). The core itself is made up of a variety of crystalline rocks, including Mesoproterozoic ortho- and paragneisses, Mesozoic granitoids, and Paleogene suites of dikes and larger intrusive bodies (Anderson et al., 1988; Anderson and Rowley, 1981). Over a large part of the core, these rocks show strong to intense ductile deformation, with a mylonitic foliation, a strong NE-trending stretching lineation, and a widely developed NE-directed sense of shear indicators (Davis et al., 1986). The detachment is overlain in part by slices of similar crystalline rocks, but lacking the mylonitic overprint, and by thick sequences of strongly faulted Miocene volcanic and sedimentary rocks (Davis et al., 1980).
Field and thin section photographs from the Whipple
Mountains. Locations are shown in Fig. 2a and listed in Table 1. All thin
sections are oriented with NE to the right.
The timing of mylonitization in the Whipple footwall is constrained to the
late Oligocene by U-Pb dating of zircons within syn-kinematic tonalitic dikes
of the Chambers Wells dike swarm. Wright et al. (1986) obtained ages of 26
The bulk of the mylonitic gneisses and granitoids that make up the core show
evidence for deformation and dynamic recrystallization under relatively high
temperature (
On the western side of the WMCC, the mylonitic foliation dips moderately
west, and disappears beneath non-mylonitic footwall rocks along a surface
described by Davis and Lister (1988) as the “mylonitic front”
(Fig. 5b). The detachment here dips more gently than the mylonitic front,
and can be traced westward to a breakaway where it reached the Miocene
ground surface. The mylonitic front, on the other hand, can be traced
seismically in the subsurface (Wang et al., 1989), where it
descends to a depth of
If the mylonitic front marks the position of the LDT at an early stage of exhumation in the Whipple Mountains, we should expect the crystalline rocks above it to show evidence of more localized ductile deformation. This is in fact the case: gneisses and granitoids above the mylonitic front show narrow (a few meters wide) shear zones with microstructures indicating that they formed under lower temperatures and higher stresses than the rocks below the mylonitic front (Fig. 5c).
On the eastern side of the range, several packages of mylonites below the
brittle detachment show evidence for deformation at lower temperature and
higher stress than the main body of mylonitic gneiss. This is evidenced by
chloritization of biotite, brittle deformation in feldspar porphyroclasts,
smaller dynamically recrystallized grain sizes, Regime 2 and locally Regime
1-type microstructures, and low temperatures from Ti-in-quartz
thermobarometry (Behr and Platt, 2011). Shear stresses determined
from these mylonites are in the range 10–42 MPa. These microstructures are
commonly associated with outcrop scale shear zones or shear bands cutting
the older, higher-temperature mylonitic foliation (Fig. 5d) or with distinctive
planar brittle–ductile shear zones referred to as mini-detachments (Fig. 5e; Axen and Selverstone, 1994; Luther et al., 2013; Selverstone et al.,
2012). Mini-detachments generally lie sub-parallel to the principal slip
surface, cut mylonitic rocks in the detachment footwall, and are associated
with narrow zones of high ductile strain with microstructures indicating
shear stresses up to
Rocks affected by this low-temperature, high-stress deformation commonly
show composite microstructures with evidence for earlier, coarser grained
microstructures overprinted by one or more stages of progressively
lower-temperature and higher-stress deformation (Fig. 5f). The bands of
high-stress mylonite may individually be only a few tens to hundreds of
microns thick, but together they can occupy a zone up to
The detachment itself is typically marked by a very sharp discontinuity, forming a polished and lineated brittle fault surface. In places, the geological evidence suggests that this surface formed very late in the history of the core complex, as it truncates structures in both the hanging wall and footwall (Davis et al., 1980). Beneath the fault is a layer of indurated cataclasite (microbreccia) that commonly forms a distinctive resistant ledge. This passes down into several meters to tens of meters of heavily altered and brecciated footwall rock (chloritic breccia), which in some areas is cut by extensive detachment-parallel faults delineated by pseudotachylite layers up to 2 cm thick (Fig. 5h).
The northern Snake Range in east-central Nevada comprises a classic domiform detachment surface (the northern Snake Range décollement, NSRD) that divides the range into a distended non-metamorphic upper plate of Paleozoic shale, limestone, and dolomite, and a ductilely deformed lower plate of metamorphosed upper Precambrian to lower Cambrian schist, quartzite, and marble intruded by Jurassic and Cretaceous granitic plutons and Cenozoic dikes (Coney, 1974, 1980; Hose et al., 1976; Miller et al., 1983; Misch, 1960; Misch and Hazzard, 1962; Fig. 2b).
Northern Snake Range footwall rocks record two phases of deformation and
metamorphism. First, a Late Cretaceous contractional event related to Sevier
thrust faulting (Miller and Gans, 1989) buried them to a depth of
Cooper et al. (2010b) put forward an alternative model for the exhumation history of the NSRD footwall in which the cooling ages and the bulk of the mylonites are unrelated to exhumation along the NSRD, and instead formed during a phase of Paleogene exhumation that predates the detachment. In this model, the mylonites represent high-strain, low-stress deformation associated with a mid-crustal LDT zone that was subsequently captured and exhumed by the brittle detachment. The dominant top-E sense of shear in the mylonites is not directly related to slip on the detachment, but reflects the overall pattern of crustal extension at the time and the regional sense of shear. Percolation of meteoric fluids into this zone of active ductile deformation would have been possible through numerous upper crustal brittle normal faults that soled into the active LDT, and need not have been related to the NSRD itself. Instead, continued thinning and cooling of the crust eventually stranded the mylonites above the active LDT, where they were captured by a moderately dipping brittle NSRD that soled down on to the active LDT (e.g., Bartley and Wernicke, 1984). As the footwall was exhumed, the NSRD flexed around a rolling hinge as an isostatic response to denudation, resulting in a subhorizontal footwall that cuts at a low angle across successive isochronal surfaces in the stranded mylonitic sequence.
Field and thin section photographs from the northern Snake
Range. Locations are shown in Fig. 2b and listed in Table 1. All thin
sections are oriented with ESE to the left.
For this scenario to be correct, we should expect to see the mylonite zone
and brittle detachment separate in the up-dip direction at a mylonitic
front, as seen in the Whipple Mountains. This relationship is not seen in
the Snake Range, in part because it has been obscured by continued
exhumation, as described above, and in part because critical relationships
are hidden beneath hanging wall rocks on the west side of the range. It is
nevertheless striking that high-strain mylonitic deformation decreases in
intensity from E to W and disappears completely in the NW part of the range
(Fig. 2b; Lee et al., 1987; Miller et al., 1983).
Non-mylonitic quartzite on the western side has a grain size up to several
hundred microns and annealed microstructures that predate a suite of 37 Ma
rhyolite dikes (Lee and Sutter, 1991), and is probably related to Late
Cretaceous metamorphism. These microstructures have subsequently been
overprinted by a small increment of deformation, producing Regime 1 (BLG II)
microstructures with a dynamically recrystallized grain size of
Sample and field photo locations.
Over a large part of the northern Snake Range, the immediate footwall to the
NSRD is dominated by a
Close to the NSRD, however, a much narrower (
We interpret these relationships to indicate that the rocks on the western
side of the range, which lay below the LDT during Late Cretaceous to Eocene
time, had already been exhumed and cooled through the LDT by the time the
detachment was initiated. This is supported by the Eocene
The Ruby Mountains–East Humboldt Range (REHR) in northeast Nevada (Fig. 1),
like the northern Snake Range, exposes the deep roots of the Cordilleran
thrust belt beneath unmetamorphosed rocks of the Cordilleran miogeocline,
separated by a complex set of detachments. The displacement direction on the
detachment system, however, is WNW (Lister and Snoke, 1984) as opposed
to ESE in the northern Snake Range. It also exposes higher-grade rocks in
its core, including large volumes of migmatitic gneiss, and some tectonic
slices of Proterozoic and Archean crystalline rocks (Lush
et al., 1988; McGrew et al., 2000). In this respect, the REHR appears to be
transitional in character with the core complexes in the northern USA and
the Canadian Cordillera, several of which have migmatitic cores (e.g.,
Brown and Murray Journeay, 1987; Parrish et al., 1988; Whitney et al., 2013
and references therein). From the deepest rocks exposed upwards, the REHR
shows the following rheological elements:
Much of the northern REHR is occupied by an assemblage of Proterozoic
and locally Archean crystalline rocks, Neoproterozoic through Paleozoic
metasedimentary rocks, and Mesozoic to Paleogene granitoid rocks, all of
which have been metamorphosed to upper amphibolite facies, accompanied by
extensive partial melting and the emplacement of a variety of dikes and
minor intrusions (Henry et al., 2011; McGrew et al., 2000; Sullivan and
Snoke, 2007). Peak metamorphic conditions of The high grade core is cut by 29 Ma biotite monzogranite dikes that
post-date the high-grade metamorphism and ductile deformation but pre-date
extensional exhumation (MacCready et al., 1997). At high structural
levels, both the dikes and the surrounding gneisses were affected by a
relatively low-temperature ductile overprint, which caused plastic
deformation and dynamic recrystallization in quartz, accompanied by
significant grain-size reduction. Above Angel Lake in the East Humboldt
Range, this deformation produces a composite microstructure (Fig. 7b) that
dies out downwards over At Secret Pass, a zone of high-strain mylonite and ultramylonite a few
tens of meters thick lies between the main detachment above (which separates
it from unmetamorphosed cover rocks), and a detachment below (separating it
from the zone of mylonitic schist and gneiss; Henry et al., 2011).
The lower detachment is probably a splay from the main detachment, which has
excised part of the sequence. These rocks have dynamically recrystallized
quartz grain sizes of The main detachment in the REHR formed under brittle conditions and is
marked by a zone of fault gouge and sharp truncations of tilted bedding in
the overlying sedimentary sequences (Henry et al., 2011). Small-scale
brittle to semi-brittle discontinuities within the mylonites probably formed
during the transition from ductile to brittle deformation during cooling and
exhumation (Fig. 7f).
Field and thin section photographs from the Ruby
Mountains–East Humboldt Range. Locations are shown in Fig. 2c and listed
in Table 1. All images are oriented with WNW to the right.
This structural sequence is consistent with those in both the northern Snake Range and the Whipple Mountains, and suggests progressive strain localization accompanying cooling and exhumation, with higher overall levels of stress in the younger, lower-temperature parts of the system. As noted above, mylonitic deformation dies out downwards beneath the detachment, and the bulk of the high-grade core (colored blue in Fig. 2c) lacks both a mylonitic foliation and lineation. This suggests that we are seeing the lower boundary of a localized zone of ductile deformation. The mylonites therefore formed above the early Miocene LDT, and represent the ductile downward extension of the brittle detachment. It appears that there was insufficient displacement on the detachment to exhume the LDT.
The high-grade gneisses in the core of the REHR formed during Mesozoic Cordilleran contraction, followed by Paleogene exhumation and cooling. The lack of evidence for strain localization suggests that deformation happened well below the LDT at that time, but the upper boundary has not been preserved. The significantly different kinematics of deformation prior to the 29 Ma biotite monzogranite dike suite, and the sharply different conditions of deformation between the high-grade gneisses and the later mylonites, suggest that the Paleogene stage of exhumation was not related to the detachment.
As in the northern Snake Range, there is considerable debate about the
timing of initiation and motion on the main detachment in the REHR.
Our proposed evolution of the REHR can therefore be summarized as follows: (1) high-grade gneisses in the core of the range show high but distributed ductile strain, which occurred before 36 Ma, below the contemporary LDT. This deformation predated emplacement of the 29 Ma monzogranite sheets, and was unrelated to the regional detachment, the sub-detachment mylonites, or the core complex itself. Stretching lineations are weak with variable orientations, but generally trend N–S. (2) The sub-detachment mylonites post-date the monzogranite sheets, and stretching lineations trend WNW. By the time this deformation occurred, in the early to middle Miocene, the gneisses had cooled and were too strong to deform in a distributed fashion when the detachment was initiated. The LDT had effectively moved down through the crust by this time, and the detachment-related mylonites formed above the LDT. (3) The REHR therefore provides a useful example of why we need to be careful in attributing deformed rocks to a particular structural or rheological level, because in an exhuming and cooling environment the LDT and the BDT will move down through the crust.
The structural analysis of three Cordilleran metamorphic core complex footwalls presented here demonstrates that distinct rheological transitions are common to all three. This implies that, despite complications of lithology or pre-existing structure, core complexes offer a consistent window into the rheological structure of the middle crust.
We relate our observations of two spatially and temporally distinct zones of ductile deformation in all three core complexes (high-stress mylonite and ultramylonite genetically related to the detachment, and high-strain, low-stress rocks that formed in the middle to lower crust) to exhumation of the footwalls through two rheological transitions: a localized–distributed transition (LDT) and a brittle–ductile transition (BDT). We suggest that moderate- to high-angle brittle normal faults in the upper crust persist as low-angle ductile shear zones below the BDT in the middle crust, soling into the LDT at greater depths.
This interpretation is consistent with a number of observed structural
relationships. In the Whipple Mountains footwall, the high-strain,
low-stress mylonitic foliation departs from the detachment on the western
side of the range, resulting in a “mylonitic front”. Above this mylonitic
front, but below the Whipple detachment, several high-stress localized
ductile shear zones are documented, consistent with exhumation of the
footwall from the LDT up through the BDT. In contrast, on the eastern side
of the range a
In the northern Snake Range, we do not see the mylonite zone depart from the detachment like it does in the Whipple Mountains, but the strength of the mylonitic fabric decreases from E to W and disappears completely in the NW part of the range. Non-mylonitic rocks from the western side exhibit high-temperature, low-stress microstructures that predate a suite of rhyolite dikes dated at 37 Ma (Lee and Sutter, 1991), whereas the rest of the range contains mylonitic schists and quartzites with high-strain, intermediate-stress microstructures that postdate the dikes. We interpret this change in deformation style to reflect exhumation and preservation of a mid-crustal LDT. Petrological evidence from rocks beneath this LDT suggests that it was originally subhorizontal in the middle crust, and was subsequently captured by the moderately dipping brittle northern Snake Range décollement that soled down into it.
The Ruby Mountains–East Humboldt Range shows a similar structural sequence of high-grade rocks overprinted by lower-temperature ductile deformation beneath the main detachment, but there is no equivalent to the mylonitic front. The mylonitic rocks in the REHR form a zone several hundred meters thick beneath the detachment, and appear to be related to the detachment. The lower boundary of the mylonites is the lower margin of a localized ductile shear zone that represents the down-dip extension of the detachment, below the BDT but above the LDT.
The differences among the three core complexes reflect (1) variations in footwall lithological assemblage, which controls the degree of strain localization during low-temperature ductile deformation; (2) the thermal structure of the crust during extensional deformation, which controls its rheological behavior; and (3) the amount of exhumation, which appears to increase from the Whipple Mountains, through the northern Snake Range, to the Ruby Mountains–East Humboldt Range.
These rheological transitions are not just limited to the Cordilleran
metamorphic core complexes, and can be found in other exposures of the
exhumed middle-to-lower crust. The Betic Cordillera in the western
Mediterranean, for example, hosts an intracontinental subduction complex in
which upper crustal rocks were buried and then exhumed from depths of
Why do we see these transitions in mechanical and rheological behavior through the crust? The BDT has long been thought to mark a depth within the crust at which the stress required for pressure-sensitive frictional deformation reaches that required for temperature-sensitive ductile creep mechanisms such as intracrystalline plasticity (e.g., Sibson, 1983). In recent years, however, it has become clear that slip on faults in the upper crust can occur at remarkably low values of static shear stress (e.g., Lockner et al., 2011). This is supported by the evidence from core complexes that the normal faults bounding them were active at very gentle dips (e.g., Davis, 1988; Davis et al., 1980; Scott and Lister, 1992). Behr and Platt (2014) suggested that the BDT may therefore be controlled by the upper temperature limits of the weakening processes affecting brittle faults, which include the formation of gouge zones occupied by smectite clays with very low frictional strengths, and/or decreases in the effect of pore fluid pressure on fault weakening (e.g., Hirth and Beeler, 2015). Observations from core complexes demonstrate that the detachment faults are indeed occupied by clay-rich gouge zones (Haines and van der Pluijm, 2008), but the lowest level in the brittle fault zone may be occupied by cataclastic breccias that lack clay and instead are cemented by hydrothermal minerals such as quartz, albite, and epidote, which have much higher frictional strengths (e.g., Selverstone et al., 2012). The occurrence of pseudotachylite veins across the BDT also supports high frictional strength at this level.
The LDT marks a level in the crust where ductile strain localization becomes
weak or absent. The width of a ductile shear zone is an indicator of the
degree of localization, and at the LDT the width becomes greater than the
scale at which we can observe it (multiple kilometers), and may extend to
the lower crust or the Moho. Our observations suggest that this transition
may be related to microstructural processes within the shear zone. At low
temperatures, crystal-plastic deformation in quartz and brittle deformation
in feldspar and micas cause substantial grain-size reduction, and hence a
transition into grain-size-sensitive creep. The latter includes processes
such as dislocation-accommodated grain-boundary sliding (DisGBS; e.g., Hirth and Kohlstedt, 2003), and dislocation creep assisted
by dynamic recrystallization (DRX creep; Platt and Behr, 2011a). These
can result in several orders of magnitude of weakening (defined by the
increase in strain rate at a given stress) and hence substantial strain
localization. At temperatures of around 500
Two distinct rheological transitions have been identified in the footwalls of three Cordilleran metamorphic core complexes: a localized–distributed transition (LDT) and a brittle–ductile transition (BDT). Their common occurrence suggests that they are ubiquitous features of the crust. As the footwalls are exhumed to the surface, they pass through first the LDT and then the BDT, resulting in the development of two zones of ductile deformation: (1) a broad zone of high-strain rock that formed in the middle crust below the LDT, overprinted by (2) a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment. In some examples (e.g., the Whipple Mountains) the LDT and the underlying high-strain rocks are spatially separate from the detachment, although they were subsequently exhumed along it. The two zones of ductile deformation show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and show superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization.
The data used to calculate the strain rate vs. depth and shear zone width vs. depth plots in Fig. 3 were obtained from Behr and Platt (2011), Hirth et al. (2001), Platt and Behr (2011b), and Rybacki et al. (2006). U-Pb zircon data for the Chambers Wells dikes are provided in Table S1. All sample and field locations referred to in the paper are listed in Table 1.
The authors declare that they have no conflict of interest.
This research was supported in part by NSF grant EAR-0809443 awarded to John P. Platt. We are grateful to Rita Economos for her assistance with the U-Pb zircon analyses and to Bernhard Grasemann, Simon Wallis, and an anonymous reviewer for their helpful and constructive reviews. Edited by: B. Grasemann Reviewed by: B. Grasemann, S. Wallis, and one anonymous referee