SESolid EarthSESolid Earth1869-9529Copernicus GmbHGöttingen, Germany10.5194/se-5-1319-2014Microscale strain partitioning? Differential quartz crystallographic
fabric development in Phyllite, Hindu Kush, Northwestern PakistanLarsonK. P.kyle.larson@ubc.cahttps://orcid.org/0000-0002-1850-1896LammingJ. L.FaisalS.Earth and Environmental Sciences, University of British Columbia, Okanagan, 3247 University Way, Kelowna, BC V1V 1V7, CanadaDepartment of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, CanadaK. P. Larson (kyle.larson@ubc.ca)16December2014521319132712August20143September201410November201419November2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.solid-earth.net/5/1319/2014/se-5-1319-2014.htmlThe full text article is available as a PDF file from https://www.solid-earth.net/5/1319/2014/se-5-1319-2014.pdf
Spatially referenced quartz c axis fabrics demonstrate the preservation of
multiple, distinct fabrics in a specimen collected from northwestern
Pakistan. The overall fabric yielded by the specimen is dominated by a single
population of quartz grains, while the fabric signatures of two other unique,
spatially distinct populations are overwhelmed. It is these minor fabrics,
however, that provide information on temperature of deformation
(403 ± 50 ∘C), differential stress (8.6 + 2.6/-1.5 MPa
to 15.0 +3.8/-2.5 MPa), strain rate (10-16 s-1 to
10-15 s-1), and strain partitioning recorded by the specimen.
Introduction
Crystallographic analysis has been long employed to study the strain
histories recorded by rock forming minerals (e.g. Turner, 1942; Sander, 1950;
Bouchez and Pêcher, 1976; Zhang and Karato, 1995). While investigation of
crystallographic fabrics have been successfully carried out on a wide variety
of mineral phases, quartz has been one of the most common targets to
elucidate strain within continental crust due to its near ubiquity in such
rocks. The development of crystallographic fabrics in quartz has been
actively investigated (e.g. Lister and Williams, 1979; Schmid and Casey, 1986)
and utilized in studies of geologic material (e.g. Bouchez and Pêcher,
1976; Blumenfeld et al., 1986; Law et al., 1990, 2004, 2011, 2013; Xypolias
and Koukouvelas, 2001; Larson and Cottle, 2014) during the past 5 decades.
While advances in our understanding and implications of the fabrics have
advanced, so too have the methods available to extract lattice orientation
data. Universal stages are still employed to generate quartz c axis
crystallographic fabrics (e.g. Kile, 2009); however, more technical methods
such as x-ray goniometry (e.g. Wenk, 1985) and electron backscattered
diffraction (EBSD) (e.g. Prior et al., 1999) can potentially provide a higher
density of information and orientation data for secondary axes. In addition,
techniques utilizing EBSD and automated optical fabric analysers (e.g.
Heilbronner and Pauli, 1993; Feuten and Goodchild, 2001; Wilson et al., 2003,
2007; Pajdzik and Glazer, 2006) have the advantage of producing spatially
referenced data with the ability to automatically generate
achsenverteilungsanalyse (AVA or axial distribution diagrams) (e.g. Sander,
1950). Such a diagram, essentially a map of crystallographic orientation
within the specimen analysed, can help facilitate the investigation and
comparison of spatially distinct grains, groups of grains, or zones within a
specimen. Spatially referenced crystallographic fabrics also allow for the
investigation of strain recorded in grains of various sizes, the potential
effects of matrix phases, and the spatial positioning of grains adjacent to
local features such as porphyroclasts.
One significant application of spatially referenced crystallographic fabric
data is to examine within-specimen fabric orientation heterogeneities. This
type of analysis has been employed to distinguish between preferred
orientations in new, recrystallized grains vs. relict porphyroclasts (e.g.
Law et al., 2010) and to identify variable dissolution in quartz veins
(Wilson et al., 2009). Such studies highlight the potentially significant
differences in crystallographic fabric development for distinct grain
populations and/or spatially separated areas of a single specimen.
This study presents new, spatially referenced crystallographic fabric data
from a specimen collected in the Chitral region of northwestern Pakistan.
This specimen records three distinct quartz crystallographic fabrics that
could be related to differences in spatial position, recrystallized grain
size, and interaction with matrix phases in the specimen. The existence of
different crystallographic fabrics that could be related to significant
changes in the texture and/or mineralogy of spatially restricted areas of a
specimen may provide insight into strain partitioning at the microstructural
scale. Moreover, the existence of distinct crystallographic fabrics at the
thin section scale has implications for the representation of strain for a
specimen using a single fabric and potentially for assessing relative
differences between spatially separated specimens.
General geology map of the Garam Chasma/Chitral region, NW Pakistan
(after Faisal et al., 2014). Specimen collection location is indicated. Field
area location is shown in regional scale inset map.
Geological setting
The Chitral region is located within the eastern Hindu Kush of northwestern
Pakistan (Fig. 1). The geology of the area is dominated by Paleozoic
protoliths, mainly low-grade metasedimentary rocks that locally reach
sillimanite grade (Gaetani et al., 1996; Zanchi et al., 2000; Hildebrand et
al., 2001; Zanchi and Gaetani, 2011; Faisal et al., 2014). These
metasedimentary rocks are intruded by a series of plutonic bodies that range
in age from Paleozoic (Kafiristan – 483 ± 21 Ma; Debon et al., 1987),
through Mesozoic (Tirich Mir – 114 to 121 Ma, Desio, 1964; Hildebrand et
al., 2000; Heuberger et al., 2007), to Cenozoic (Garam Chasma – 24 Ma;
Hildebrand et al., 1998). The region records a protracted deformational
history with the earliest records indicating Late Triassic deformation and
metamorphism and recent events culminating in the Early Miocene (Faisal et
al., 2014).
Thin section scale photomicrographs of specimen S32 presented
in plane-polarized light (a), cross-polarized light (b), and as an AVA
diagram (c). The location of quartz grains analysed is indicated by different
coloured and shaded circles in (c). White circles denote a coarser grain within
the quartz-rich lens; black circles indicate a finer grain within a
quartz-rich lens; yellow circles mark a matrix quartz grain measured. A more
detailed cross-polarized photomicrograph of the quartz-rich lens is shown in
(d); coarser and finer populations are marked.
Specimen S32, the subject of the present study, is part of a suite of
quartz-rich specimens collected in the Chitral region. It is a quartz +
muscovite + chlorite phyllite (Fig. 2a, b) collected between the Tirich Mir
and Reshun faults (Fig. 1). The foliation in the specimen is defined by
planar muscovite and chlorite laths while the lineation is defined by a
grain-shape fabric of the same minerals. A thin section of specimen S32 was
cut parallel to the macroscopic lineation (25∘→006∘) and
perpendicular to the foliation (330∘/38∘ NE). The specimen has
a heterogeneous mineral distribution with localized quartz-rich lenses
(Fig. 2a, b) that have a bimodal grain-size distribution (Fig. 2d). The
coarser grain population within the lens has a median area (as calculated for
an ellipse using the long and short axes of each grain) in this section of
161 µm2 with a standard deviation of 45 µm2 and an aspect ratio
of 2.5 (standard deviation of 1.0). The smaller grain-size population within
the quartz-rich lens is characterized by a median area of
81 µm2 with a standard deviation of 20 µm2 and an aspect ratio of
2.3 (standard deviation of 1.0). The long axes of both grain-size populations
are typically at low angles relative to the dominant foliation. The
quartz-rich lenses are surrounded by phyllosilicate-rich layers that contain
quartz grains with a median elliptical equivalent surface area of
52 µm2 with a standard deviation of 13 µm2 and an aspect ratio of
2.0 (standard deviation of 0.7). These grains are typically elongate parallel
to the foliation direction. The crystallographic fabrics of each quartz grain
population are investigated below.
Method
The specimen was oriented during collection and cut parallel to macroscopic
lineation and perpendicular to the macroscopic foliation. The orientations of
c axes within the specimen were determined using a G50 Automated Fabric
Analyser (e.g. Wilson and Peternell, 2011) with an RGB filtered, colour charge-coupled
device (CCD) sensor and white LEDs at an optical resolution of 10 µm. Previous
research has shown that c axis orientations determined using an automated
fabric analyser like the G50 are indistinguishable from those determined
using X-ray (Wilson et al., 2007) and EBSD methods (Peternell et al., 2010).
The G50 outputs an interactive AVA diagram (Fig. 2c), or c axis map, of the
thin section that was used to build crystallographic fabrics. Because each
pixel of the AVA diagram has unique c axis orientation data associated with
it, the crystallographic fabrics of spatially distinct sections within the
specimens can be investigated by picking the exact locations within grains
from which the orientation data are to be extracted.
The existence of three spatially and texturally distinct quartz grain-size
populations within the specimen allows the direct investigation of potential
microscale quartz crystallographic fabric and strain differences. Such
investigations allow assessment of the sense of shear recorded by the
different populations and the slip systems active during fabric formation.
Moreover, the different grain-size populations lend themselves to
paleopiezometric investigation through the application of the Stipp and
Tullis (2003) paleopiezometer as modified by Holyoke and Kronenberg (2010).
These paleopiezometric estimates, in turn, can be combined with derived
deformation temperatures to estimate strain rates. The results from this
study have bearing on microscale strain, stress, and strain rate partitioning
during deformation and on the potential homogenizing effects of dominant
grain-size populations in crystallographic fabric data, which may obscure
contributions from other smaller populations.
Quartz microstructures
In the equal area stereonets used to present the c axis data, the lineation
lies horizontally across the equator, while the foliation is a vertical plane
cutting through the equator. The stereonets are oriented such that a dextral
asymmetry indicates top-to-the-east-southeast shear.
Quartz microtextures observed in thin sections. All
photomicrographs are cross polarized light. (a) Three examples of minor
bulging recrystallization. (b) Subgrain development within the quartz-rich
lens. Also visible are deformation lamellae. (c) Same location as in (b) with
the stage rotated to further highlight subgrain formation. (d) A matrix quartz
grain (centre) encased by phyllosilicates.
Quartz microstructures
The quartz grains that comprise the finer and coarser populations within the
quartz-rich lens in the specimen demonstrate textural characteristics
consistent with dynamic recrystallization. In both populations there is
evidence of minor bulging (Fig. 3a), subgrain development (Fig. 3b, c), and
deformation lamellae (Fig. 3b, c). These textures are most consistent with
Regime 2 crystallization of Hirth and Tullis (1992) or the SGR (subgrain rotation recrystallization) category of
Stipp et al. (2002).
Quartz crystallographic fabrics from various quartz
populations in the specimen. All diagrams are lower hemispherical equal area
stereonet projections contoured at 1 % intervals. Contours for (a) are 1, 2,
3, 4 times uniform; for (b) through (d) they are 1, 2, 3, 4, 5, 6+ times
uniform. The stereonets are oriented such that the foliation forms a vertical
plane, while the observed lineation (and orientation of thin section) follows
a horizontal E–W line. (a) Combined/bulk crystallographic fabric generated
from an 8000-point grid mapped across the specimen. (b) Quartz
crystallographic fabric generated from manually selected matrix grains. (c)
Crystallographic fabric of the finer-sized quartz population within the
quartz-rich lens. (d) Crystallographic fabric of the coarser-sized quartz
population within the quartz-rich lens.
In contrast, strong evidence for dynamic recrystallization was not observed
in the quartz grains found within the phyllitic matrix outside of the
quartz-rich lens. Here, the grains are commonly partially surrounded by
muscovite and/or chlorite laths (Fig. 3d) and as such typically have
restricted contact with one another.
Quartz crystallographic fabric results
When examined in bulk (i.e. looking at fabric automatically generated from a
non-discriminant sampling grid) specimen S32 yields a crystallographic fabric
consistent 〈a〉, prism 〈a〉, and prism [c]
slip systems (Schmid and Casey, 1986; Fig. 4a). There is a slight asymmetry
in the basal 〈a〉 fabric that is consistent with top-to-the-east-southeast shear. If the crystallographic fabrics of the three
differently sized quartz grain populations are examined individually, however, it becomes
apparent that the overall or bulk crystallographic fabric is dominated by
the matrix quartz population.
The crystallographic fabric yielded from the matrix quartz bears a strong
resemblance to the bulk fabric (Fig. 4b). While showing similar activation of
the prism 〈a〉 and prism [c] slip systems, the matrix quartz
c axis fabric indicates preferred activation of the rhomb 〈a〉 slip system over basal 〈a〉. Moreover, in the
hand-picked pattern there appears to be a stronger prism 〈a〉
component and a more well-defined rhomb 〈a〉 asymmetry
(top to the east-southeast). The prism [c] positions also appear to define
an asymmetry, but it yields the opposite shear sense to that indicated by the
basal 〈a〉 fabric (Fig. 4b).
In contrast to both the bulk and the matrix grain-size population, the fabric
yielded by the finer-size population within the quartz lens comprises a
single girdle with activation of the prism 〈a〉 and rhomb
〈a〉 slip systems (Fig. 4c). There is no indication of prism
[c] activation. The single girdle is inclined to the right, which is
consistent with top-to-the-east-southeast shear.
The crystallographic fabric from the coarser grain-size population in the
lens is similar to that from the finer-sized population; activation of the
prism 〈a〉 and rhomb 〈a〉 slip systems
dominates. Unlike the other intra-lens population, however, the fabric of the
coarser-sized grains forms a type-1 crossed-girdle (Fig. 4d). The main fabric
displays a top-to-the-right (or southeast) asymmetry, with weakly developed
secondary arms extending away from the main girdle (Fig. 4d).
Quartz crystallographic fabric interpretation
With the exception of the prism [c] slip (discussed below) the fabric
asymmetries noted in the various specimen populations are consistent with
interpreted top-to-the-east-southeast movement across the nearby Tirich Mir
and Reshun faults (Fig. 1; Calkins et al., 1981; Hildebrand et al. 2001).
The quartz crystallographic fabric from the smaller grain-size population in
the specimen analysed indicates a component of prism [c] slip. Slip in the
prism [c] direction is typically associated with deformation in excess of
600–650 ∘C (Lister and Dornsiepen, 1982; Mainprice et al., 1986;
Morgan and Law, 2004). The rock sampled, however, is a low-metamorphic-grade
phyllite and has not experienced temperatures in the range of those expected
to favour prism [c] slip.
Similar unexpected patterns have been noted in low-metamorphic-grade slates
and phyllites in New Zealand where they are interpreted to reflect mechanical
rotation of grains elongate in the c axis direction parallel with the
stretching direction (Stallard and Shelly, 1995). Such an interpretation is
consistent with the sparse evidence of dynamic recrystallization in the
matrix quartz. However, c axis orientations consistent with slip in the
rhomb and prism 〈a〉 directions indicate that there was some
dynamic modification of the crystal lattice in response to deformation. As
suggested by Stallard and Shelly (1995), physical rotation of the clasts may
have occurred preferentially in the matrix grains surrounded by
phyllosilicate-rich layers, into which strain was preferentially partitioned.
The matrix quartz grains that occur in areas with less abundant
phyllosilicate may have accommodated more of the strain directly through
dislocation slip resulting in the development of the prism 〈a〉 and rhomb/basal 〈a〉c axis orientations observed
in the crystallographic fabric.
The development of quartz c axis maxima parallel to the stretching
lineation may alternatively be explained by preferential dissolution of
quartz grains with their (0001) planes parallel to the foliation. The
dissolution of such grains and re-precipitation and/or concentration of
residual grains with c axes parallel to the foliation have been interpreted
to account for similar c axis patterns in low-metamorphic-grade rocks in
southeastern Brazil (Hippertt, 1994).
The orientations of c axes in grains that comprise the quartz-rich lens in
the specimen appear to have been controlled by dynamic recrystallization
(Fig. 3a–c) as part of their deformational response to imposed stresses.
Because the quartz records evidence of dynamic recrystallization, the
crystallographic fabrics measured from it are interpreted to reflect the
modification of its crystal lattice orientation in response to deformation.
Deformation temperature, grain-size piezometry, and strain rate estimates
The crystallographic fabric from the coarser grains in the quartz lens forms
a weakly developed crossed-girdle fabric (Fig. 4d). The opening angle of such
fabrics, that is the angle between the arms of the fabric as measured about
the perpendicular to the flow plane, has been empirically related to the
estimated temperatures at which the fabrics developed (Kruhl, 1998; Morgan
and Law, 2004; Law, 2014). Converting a fabric opening angle into a
temperature of deformation requires a number of assumptions to be made,
including temperature being the primary control on critically resolved shear
stress, as opposed to strain rate or hydrolytic weakening. See Law (2014) for
an in depth review of the considerations in using quartz crystallographic
fabric opening angles as geothermometers. In reflection of the uncertainty in
the data used for the empirical calibration and the precision of the opening
angle determined, quartz crystallographic fabric-derived deformation
temperatures are quoted at ±50 ∘C (Kruhl, 1998).
The crossed girdle fabric in the specimen analysed has an opening angle of
∼ 53 ∘(Fig. 4d), which corresponds to a deformation temperature
of ∼ 403 ± 50 ∘C. Although the c axis fabric is weakly
developed, the temperature estimate is consistent with the interpreted
metamorphic grade of the rock and with the observed microstructures, which
are dominated by subgrain development with minor bulging. The transition from
bulging to subgrain formation processes in the eastern Tonale fault zone of
the Italian Alps is associated with temperatures near 400 ∘C (Fig. 9
of Stipp et al., 2002). Similar textures from the Himalaya may occur at
slightly higher temperatures, closer to 450 ∘C (Law, 2014). It
should be noted, however, that, as with c axis opening angles, strain rate
and hydrolytic weakening can also play an important role in the development
of quartz textures (e.g. Law, 2014).
Cross-polarized (xpl) and plane polarized (ppl)
photomicrographs of the quartz-rich lens analysed in specimen S32. Location
of photomicrographs is shown in Fig. 2d. The coarser- and finer-size
portions are approximately the same thickness and quartz grains in both do
not appear to be significantly affected by pinning of phyllosilicate
material.
Strain rate estimates for the two size-populations within the
quartz-rich lenses using the flow laws of Hirth et al. (2001) and Rutter and
Brodie (2004). Differential stress estimates are from recrystallized
grain-size piezometry while temperature estimates are from quartz
crystallographic fabric opening angles. See text for discussion.
Recrystallized grain-size piezometry as proposed by Stipp and Tullis (2003)
and recalibrated by Holyoke and Kronenburg (2010) may be used to estimate
potential differences in differential flow stresses recorded in different
dynamically recrystallized grain-size populations. Experimental calibration
of the quartz grain-size piezometer applies to bulging recrystallization
mechanisms and extends to a maximum grain size of ∼ 50 µm
(Stipp and Tullis, 2003; Stipp et al., 2006). Stipp et al. (2010) suggest
that the piezometer may be reasonably applied to grains formed through
subgrain rotation recrystallization, but would significantly underestimate
those developed during grain boundary migration recrystallization. Applying
the quartz recrystallization piezometer to the two dynamically recrystallized
grain-size populations in the quartz-rich lenses first requires assessment of
potential secondary controls on grain size, such as pinning by
phyllosilicates. Detailed examination of coarser and finer-grain-size regions
within the quartz-rich lens (see location of detailed area in Fig. 2d)
demonstrates that while there may be a minor increase in the amount of
phyllosilicate associated with the finer-grain-size region it does not
control the size of the quartz grains (Fig. 5). If dynamic recrystallization
is considered to be the primary control on grain size in the quartz-rich lens
then calculations based on the Holyoke and Kronenburg (2010) calibration of
the Stipp and Tullis (2003) piezometer indicate differential stresses of 8.6
+2.6/-1.5 and 15.0+3.8/-2.5 MPa for the coarser and finer quartz
grain-size populations, respectively.
The differential stress estimates determined can be combined with deformation
temperature and plotted atop a series of different geologically reasonable
strain rates (Fig. 6). As pressure constraints have not been established for
the specimen S32 or any relevant nearby locales, the fugacity used in both
the Hirth et al. (2001) and Rutter and Brodie (2004) quartz flow law
calibrations utilized was estimated using the derived deformation
temperature, a thermal gradient of 25 ∘C km1, and an average
crustal density of 2.85 g cm-3. The resulting fugacity, 108 MPa, was
calculated as in Pitzer and Sterner (1994). As noted in Law et al. (2013),
calculated strain rates are rather insensitive to changes in fugacity; using
a thermal gradient of 40 ∘C km1 in fugacity calculations does
not result in a significant change in the strain rate estimates for this
study. Plotted differential stresses and deformation temperature indicates a
faster strain rate for the finer grains/higher differential stress (Fig. 6).
The strain rate estimates vary considerably between the two calibrations with
only the Hirth et al. (2001) calibration providing estimates that approach
those geologically reasonable (Fig. 6).
Discussion
The size variation between the matrix and lens quartz grains in the specimen
may reflect primary differences associated with the protolith. The finer-sized quartz grains found within the phyllitic matrix are interpreted to
represent smaller grains deposited within a silt-/mud-dominated protolith,
while the coarser quartz that occurs within the specimen is interpreted to
represent a thin sand lens. Within the lens itself, the two grain-size
populations may reflect further primary differences, secondary modification
during deformation, or both. These possibilities are discussed below.
It is possible that the two grain-size populations within the lens reflect
different strain histories. The quartz within the lens has been subject to
dynamic recrystallization during which there would have been potential for
the grains to change size and shape. The grain-size difference within the
lens may reflect development of the finer population where stress was
preferentially partitioned resulting in more intense grain-size reduction,
whereas the courser population, affected by lower stresses, may reflect more
limited grain-size reduction. Such stress partitioning is consistent with
differential stress estimates made based on grain-size piezometry that
indicated higher stresses associated with smaller grain sizes.
The two grain sizes may, alternatively (or additionally), reflect an initial
difference in grain size inherited from the sand lens when it was first
deposited, perhaps compounded by incomplete recrystallization of the
larger grains. The variation in grain size within the quartz-rich lens may
represent a combination of both primary differences and secondary strain
partitioning. Finer grains within the quartz lens may have been preferred for
initial strain partitioning, which would have facilitated, and been enhanced
by, further grain-size reduction and higher strain rates. Strain
concentration within the finer grains in the quartz-rich lens is consistent
with the variation in crystallographic fabrics between the two size
populations. The coarser-grain-size fabric maintains secondary trailing arms
(Fig. 4d), whereas in the finer-grain-size fabric, those arms have been
essentially obliterated (Fig. 4c). Migration towards a single girdle fabric
has been associated with increased critically resolved shear stress and shear
strain (Keller and Stipp, 2011) in quartz crystallographic fabric evolution
models.
Conclusions
This study demonstrates the importance of spatial resolution and
registration in specimens analysed for crystallographic fabric analyses. In
this metapelite example, the bulk crystallographic fabric overwhelmed two
spatially restricted fabrics recorded in a quartz lens. Yet it was the
secondary, spatially distinct fabrics that yielded information on
deformation temperature, paleopiezometry, and strain rate. This has
important implications for increasingly common studies that examine large
numbers of specimens utilizing automated methods; care must be taken to
investigate the spatial distribution of fabric symmetry within specimens as
the bulk pattern may average and mask important information. The
spatially controlled crystallographic fabric patterns documented in this
study may reflect the fundamental initial properties of the specimen, byproducts of differential strain partitioning at the microscale, or some
combination of the two.
Acknowledgements
This project was supported by NSERC Discovery and CFI Leaders Opportunity
Fund grants to K. Larson. A. Khan and the NCEG at the University of Peshawar
are thanked for their logistical assistance during fieldwork. Discussions
with D. Kellett, reviews by C. Wilson and N. Mancktelow, and editorial
handling and initial review by R. Law have helped improve the clarity of the
manuscript.
Edited by: R. Law
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