SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-9-859-2018Inverted distribution of ductile deformation in the relatively “dry”
middle crust across the Woodroffe Thrust, central AustraliaInverted distribution of ductile deformation across the Woodroffe ThrustWexSebastianMancktelowNeil S.neil.mancktelow@erdw.ethz.chhttps://orcid.org/0000-0002-7404-321XHawemannFriedrichhttps://orcid.org/0000-0003-0440-6537CamachoAlfredohttps://orcid.org/0000-0002-8517-168XPennacchioniGiorgiohttps://orcid.org/0000-0002-5956-5327Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, SwitzerlandDepartment of Geological Sciences, University of Manitoba, 125 Dysart Rd, Winnipeg, Manitoba, R3T 2N2, CanadaDepartment of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padua, ItalyNeil S. Mancktelow (neil.mancktelow@erdw.ethz.ch)11July20189485987831January20187February201829May201818June2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://se.copernicus.org/articles/9/859/2018/se-9-859-2018.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/9/859/2018/se-9-859-2018.pdf
Thrust fault systems typically distribute shear strain preferentially
into the hanging wall rather than the footwall. The Woodroffe Thrust in the
Musgrave Block of central Australia is a regional-scale example that does not
fit this model. It developed due to intracontinental shortening during the
Petermann Orogeny (ca. 560–520 Ma) and is interpreted to be at least
600 km long in its E–W strike direction, with an approximate top-to-north
minimum displacement of 60–100 km. The associated mylonite zone is most
broadly developed in the footwall. The immediate hanging wall was only
marginally involved in the mylonitization process, as can be demonstrated
from the contrasting thorium signatures of mylonites derived from the upper
amphibolite facies footwall and the granulite facies hanging wall protoliths.
Thermal weakening cannot account for such an inverse deformation gradient, as
syn-deformational P–T estimates for the Petermann Orogeny in the hanging
wall and footwall from the same locality are very similar. The distribution
of pseudotachylytes, which acted as preferred nucleation sites for shear
deformation, also cannot provide an explanation, since these fault rocks are
especially prevalent in the immediate hanging wall. The most likely reason
for the inverted deformation gradient across the Woodroffe Thrust is
water-assisted weakening due to the increased, but still limited, presence of
aqueous fluids in the footwall. We also establish a qualitative increase in
the abundance of fluids in the footwall along an approx. 60 km long section
in the direction of thrusting, together with a slight decrease in the
temperature of mylonitization (ca. 100 ∘C). These changes in ambient
conditions are accompanied by a 6-fold decrease in thickness (from
ca. 600 to 100 m) of the Woodroffe Thrust mylonitic zone.
Introduction
Continental fault and shear zone systems (e.g. Ramsay, 1980) with
displacements on the order of several tens to hundreds of kilometres
generally show an asymmetric mylonite distribution across the main fault
horizon that is opposite for reverse faults or thrusts and normal faults or
detachments. Fault zones are predicted to become more viscous and broaden
with depth (e.g. Fossen and Cavalcante, 2017; Handy et al., 2007; Mancktelow,
1985; Passchier and Trouw, 2005; Platt and Behr, 2011b). The juxtaposition of
initially different crustal levels should therefore result in a geometry
that, for a thrust, preferentially preserves the broader ductile mylonite
zone in the hanging wall, whereas, for a detachment, it should be in the
footwall (e.g. Mancktelow, 1985, his Fig. 11; Passchier, 1984, his Fig. 2).
This model is valid for many large-scale fault and shear zone systems, for
example, the Moine Thrust Zone, NW Scotland (Christie, 1963; Coward, 1980);
the Alpine Fault, New Zealand (Cooper and Norris, 1994; Sibson et al., 1981);
the Saint-Barthélemy Massif shear zone, Pyrenees, southern France (Passchier, 1984); the Simplon
Fault, central European Alps (Mancktelow, 1985); the Grizzly Creek shear
zone, Colorado (Allen and Shaw, 2011); and the Whipple Mountains detachment
fault, south-western USA (Davis, 1988; Davis and Lister, 1988). The mid- to
lower-crustal Woodroffe Thrust of central Australia (Major, 1970) is an
example that does not fit this model and predominantly developed a broader
mylonite zone in the footwall (Bell and Etheridge, 1976; Camacho et al.,
1995; Flottmann et al., 2004). An interpretation of the Woodroffe Thrust as
an original detachment that was later re-oriented and exploited as a thrust
can be excluded, both because the metamorphic grade decreases in the
direction of tectonic transport and because field mapping shows that the
fault zone steepens and ramps down towards the internal part of the orogen,
against the transport direction (Wex et al., 2017). Passive transport and
thermal weakening also cannot account for the inverse deformation gradient,
as there is no evidence for late brittle movement on the thrust plane and
syn-deformational P–T estimates in the hanging wall and footwall from
the same locality are very similar (Wex et al., 2017). Bell and
Etheridge (1976) and Camacho et al. (1995) proposed that the inverted
distribution of ductile deformation is explained by the difference in bulk
water content between the upper amphibolite facies (1.0 wt %) footwall
and the granulite facies (0.2 wt %) hanging wall, reflecting the
metamorphic conditions in the protolith prior to thrusting. Similarly, the
preferential formation of shear zones in regions where the host rock
mineralogy had previously been modified by fluid–rock interaction has, for
example, also been documented in the Whipple Mountains detachment fault in SE
California (Selverstone et al., 2012) and the Neves area of the Tauern Window
in the Eastern Alps (Mancktelow and Pennacchioni, 2005). In this paper, we
discuss the control of host rock lithology and, potentially, of fluid
activity on the distribution of ductile deformation across the Woodroffe
Thrust, in an attempt to test the local findings of Bell and Etheridge (1976)
and Camacho et al. (1995) on a more regional scale (Fig. 1). In particular,
we utilize the radiogenic signature of footwall and hanging wall rocks to
constrain their respective degree of reworking in the ductile mylonite zone.
We also investigate the effect of varying metamorphic temperatures and fluid
conditions on the change in mylonite thickness over a distance of ca. 60 km
parallel to the direction of thrusting.
Geological map of the central Musgrave Block (modified after Major
et al., 1967; Sprigg et al., 1959; Young et al., 2002).
Geology
The mid- to lower-crustal Woodroffe Thrust outcrops in the Musgrave Block of
central Australia (Major, 1970), which is located within the suture zone of
the north, west, and south Australian cratons. These cratons were amalgamated
into the Australian continent at approx. 1300 Ma in an early stage of
development of the supercontinent Rodinia (Myers et al., 1996). The Woodroffe
Thrust is developed over an E–W strike length generally interpreted to
exceed 600 km and separates the Mulga Park Subdomain in the footwall (to the
north) from the Fregon Subdomain in the hanging wall (to the south) (Camacho and Fanning, 1995;
Edgoose et al., 1993; Major and Conor, 1993). Exposure of the thrust is poor
to inexistent in its proposed western (e.g. Stewart, 1995, 1997) and eastern
(e.g. Edgoose et al., 2004) prolongations but is generally excellent for
approx. 150 km in the central Musgrave Block (Bell, 1978; Camacho et al.,
1995; Collerson et al., 1972; Wex et al., 2017), where the current study was
conducted. In this region, both footwall and hanging wall predominantly
consist of granitoids (more common in the footwall) and quartzo-feldspathic
gneisses (more common in the hanging wall), with subordinate metadolerites,
mafic gneisses, and metapelites (Fig. 1). Rare quartzites, amphibolites, and
schists are restricted to the footwall (Camacho and Fanning, 1995; Collerson
et al., 1972; Major, 1973; Major and Conor, 1993; Scrimgeour and Close, 1999;
Young et al., 2002). Protoliths are inferred to have been felsic volcanics,
sediments, and intrusives with depositional or emplacement ages around
1550 Ma (Camacho, 1997; Camacho and Fanning, 1995; Gray, 1977, 1978; Gray
and Compston, 1978; Maboko et al., 1991; Major and Conor, 1993; Sun and
Sheraton, 1992). These protoliths were regionally deformed and metamorphosed
at upper amphibolite facies (Mulga Park Subdomain) to granulite facies
(Fregon Subdomain) conditions during the ca. 1200 Ma Musgravian Orogeny
(Camacho, 1997; Camacho and Fanning, 1995; Gray, 1978; Maboko et al., 1991;
Sun and Sheraton, 1992) and syn- to post-tectonically intruded by the
Pitjantjatjara Supersuite granitoids between ca. 1170 and 1130 Ma (Camacho,
1997; Camacho and Fanning, 1995; Scrimgeour et al., 1999; Smithies et al.,
2011). Peak temperature and pressure conditions during the Musgravian Orogeny
are controversial. Earlier estimates were ca. 820–900 ∘C and
1.1–1.4 GPa (Ellis and Maboko, 1992; Maboko et al., 1989, 1991), but more
recent studies have given conditions of ca. 800–1000 ∘C and
0.6–0.9 GPa (Camacho, 1997; Tucker et al., 2015; Walsh et al., 2015).
Subsequent to this regional-scale tectono-metamorphic phase, the area
experienced bimodal magmatism (Giles Complex, including the Alcurra Dolerite
swarm) between ca. 1080 and 1050 Ma and mafic magmatism (Amata Dolerite) at
approx. 800 Ma (Ballhaus and Glikson, 1995; Camacho et al., 1991; Clarke et
al., 1995; Edgoose et al., 1993; Evins et al., 2010; Schmidt et al., 2006;
Sun et al., 1996; Zhao et al., 1994; Zhao and McCulloch, 1993). In the area
considered in the current study, gabbro–norite intrusions, correlated to the
Giles Complex, are restricted to the Fregon Subdomain. Except for these
periods of magmatic intrusion, the central Musgrave Block remained largely
unaffected by tectonic events between the Musgravian Orogeny at ca. 1200 Ma
and the Petermann Orogeny at ca. 560–520 Ma (Camacho and Fanning, 1995;
Maboko et al., 1992). Earlier studies by Maboko et al. (1989, 1991) and Ellis
and Maboko (1992) proposed that the Musgravian and Petermann orogenies
represented successive stages of a single anticlockwise
P–T–t path, whereas Camacho (1997) and
Camacho et al. (1997, 2015) suggested that the Musgrave Block was partially
exhumed prior to the onset of the Petermann Orogeny and then reburied.
Photograph and schematic sketch of a cross section through the
Woodroffe Thrust at Kelly Hills. Ductile deformation is almost entirely
concentrated in the immediate footwall (Mulga Park Subdomain), developing a
sequence of protomylonites, mylonites, and ultramylonites, with the degree of
mylonitization decreasing into the footwall. In contrast, the mostly
unaffected or only weakly foliated hanging wall (Fregon Subdomain) is
characterized by ubiquitous and voluminous pseudotachylyte (pst) veins and
breccias. Further into the hanging wall, a dip slope, characterized by a
second zone of highly abundant unsheared pseudotachylyte, has also been
documented. Photograph coordinates: 131.45077, -25.89823.
The Petermann Orogeny produced a number of large-scale mylonitic shear zones,
amongst which the Woodroffe Thrust is the most prominent. Ductile deformation
during top-to-north thrusting along the Woodroffe Thrust was largely
accommodated within the Mulga Park Subdomain footwall (Bell and Etheridge,
1976; Camacho et al., 1995; Flottmann et al., 2004) and is characterized by
mylonites with varying degrees of strain, ranging from protomylonites to
ultramylonites (Fig. 2), anastomosing around low-strain domains on the metre
to kilometre scale. The syn-kinematic conditions of deformation along the
Woodroffe Thrust were constrained by conventional geothermobarometry to be
within the range of 520–650 ∘C and 0.8–1.3 GPa (Wex et al., 2017)
and thus of lower temperature when compared to peak conditions during the
earlier Musgravian Orogeny. The metamorphic assemblage in both mylonitic
samples (where the metamorphic aggregate developed syn-kinematically) and in
statically overprinted undeformed samples (e.g. some Alcurra and Amata
Dolerite dykes) record similar metamorphic conditions, consistent with
ambient mid- to lower-crustal levels (Wex et al., 2017). Stable mineral
assemblages during the Petermann Orogeny in felsic units of the Woodroffe
Thrust comprise (decreasing modal abundance from left to right)
Qz + Pl + Kfs + Bt + Ilm ± Grt ± Ep ± Ms ± Ky ± Cpx ±
Hbl ± Rt ± Ttn ± Mag ± Cal, whereas mafic units
consist of
Pl + Ilm ± Cpx ± Grt ± Rt ± Opx ± Bt ± Hbl ± Ky ± Mag
± Qz ± Kfs ± Cal (Wex et al., 2017). Mineral abbreviations
are after Whitney and Evans (2010). The degree of mylonitization
progressively decreases into the footwall but shows a very abrupt transition
into the immediate, dominantly brecciated hanging wall (Figs. 2, 3), which is
characterized by ubiquitous and voluminous pseudotachylyte veins and breccias
(Camacho et al., 1995; Lin et al., 2005). Even though this upper boundary of
the mylonites is discrete or rapidly transitional in the field (Fig. 3), it
does not necessarily represent the original boundary between the Mulga Park
(footwall) and Fregon (hanging wall) subdomains. In the
Amata area (western edge of Fig. 1), Bell (1978) reported up to 250 m of
marginal hanging wall reworking into the mylonite zone. However, it remains
uncertain how this value was exactly determined, since hanging wall and
footwall mylonites are very similar in their field appearance.
Methods and general approach
The hanging wall, footwall, and numerous transects across the Woodroffe
Thrust have been studied and sampled along a N–S traverse, parallel to the
direction of thrusting. The sampling locations are reported in Fig. 1. Thin sections of the sampled
mylonites were prepared from rock chips cut perpendicular to the foliation
and parallel to the stretching lineation and analysed using standard
polarized light and scanning electron microscopy. Firstly, the distribution
of ductile deformation along and across the Woodroffe Thrust was
characterized by quantifying (1) the regional variation in the maximum
thickness of the mylonitic zone and (2) the associated degree of hanging wall
and footwall reworking. Secondly, field and thin-section observations were
compiled to assess in a qualitative manner (3) the presence/absence of fluids
during deformation and (4) the regional variability in modal abundance of
hydrous minerals in felsic units. Potential correlations between
parameters (1) to (4) are then discussed. Sample/outcrop coordinates are
given in the world geodetic system (WGS) 1984. Orientation measurements of
structural elements are corrected for magnetic declination. A detailed
description of all utilized methods is given in Supplement Sect. S1.
Sharp contact between the ultramylonites of the Woodroffe Thrust
(below) and the largely undeformed felsic granulite in the hanging wall
(above). Photograph looking perpendicular to the direction of thrusting.
Outcrop SW13-135 (coordinates: 131.87939, -26.21188; location 12 in
Fig. 1).
Mylonite thickness
Field observations indicate that the thickness of the Woodroffe Thrust
mylonites is variable. In a section perpendicular to strike, the thickness
(T) of the mylonitic zone across the Woodroffe Thrust was calculated by
trigonometry (Fig. 4) from (1) the angle of dip (α) of the thrust
(measured in the field and averaged for each transect); (2) the respective
difference in elevation (p) (derived from the 30 × 30 m digital elevation model
ASTER) between the lower and upper structural boundaries of the mylonites to
the unsheared country rocks (determined from field observations and remote
sensing); and (3) the apparent thickness (q) (derived from remote sensing).
The upper boundary of the mylonitic zone is easily recognized (Fig. 3). The
lower boundary is determined by the first appearance of Petermann mylonitic
foliation, well characterized by the approximate N–S trend of the stretching
lineation and by the top-to-north kinematic indicators. Errors for
parameters α and q are considered negligible, whereas the 30 × 30 m
resolution of the digital elevation model is prone to introduce an
uncertainty on the order of 10–20 m. The geometrical arrangement for the
estimate of T was the same for locations 2–8 and 12–14 (Fig. 4a) but slightly different for location 11 (Fig. 1), where the lower structural
boundary of the mylonites was at a higher elevation than the upper boundary
(Fig. 4b). The thickness of the mylonitic zone at location 1 (Fig. 1) was
not calculated because the Woodroffe Thrust is only exposed along-strike.
The results for all other studied transects are summarized in Table 1 and,
disregarding local variability, indicate a gradual increase in mylonite
thickness from ca. 100 m in the north to ca. 600 m in the south.
Schematic illustration of the trigonometry applied to quantify the
true thickness of the Woodroffe Thrust mylonitic zone in a section
perpendicular to strike. The parameters are defined in the main text.
(a) Geometry applicable to locations 2–8 and 12–14 of Fig. 1.
(b) Geometry applicable to location 11 of Fig. 1.
Airborne thorium (ppm) map of the central Musgrave Block. Data
derived from the Australian Geophysical Archive Data Delivery System (GADDS)
under http://www.ga.gov.au/gadds (last access: November 2013)
(Percival, 2010). The hanging wall Fregon Subdomain is generally depleted
compared to the footwall Mulga Park Subdomain. Granitoid intrusives of the
Pitjantjatjara Supersuite often have a higher thorium content, particularly
in the footwall (one example surrounded by the blue dashed line) but also in
the southern part of the hanging wall. Lower-thorium signatures forming
diffuse tongues extending northward into the footwall are attributed to
alluvial wash derived from the hanging wall. The distinction in thorium
content is no longer evident for the northernmost klippe of the hanging wall
in the Kelly Hills area (locations 1–5) and the northernmost part of the
Mt Fraser klippe (location 7).
Degree of hanging wall and footwall mylonitization
Angle of dip (α), elevation difference (p), apparent
thickness (q), and true thickness (T) of the Woodroffe Thrust mylonites
in the central Musgrave Block.
Mylonites derived from the hanging wall and footwall of the Woodroffe Thrust
are very similar in both their field appearance and
microstructural–petrographical characteristics. The high degree of
recrystallization and general lack of porphyroclasts in the uppermost
mylonites at locations 4, 6–8, and 12–14 (Fig. 1) suggest that these
samples were potentially derived from the pseudotachylyte-rich Fregon
Subdomain, hence indicating local hanging wall reworking. To establish
whether mylonites were derived from either the Mulga Park or the Fregon
subdomains, we utilized their thorium concentrations, based on the general
observation that hanging wall rocks are depleted in thorium compared to
footwall rocks (Fig. 5). This contrast is due to (1) dehydration and melting
reactions during the earlier ca. 1200 Ma Musgravian Orogeny, which depleted
the granulite facies hanging wall to a greater degree than the upper
amphibolite facies footwall (Heier and Adams, 1965; Lambert and Heier, 1967,
1968), (2) the predominance of granitoids rather than gneisses in the
footwall, and (3) the fact that low-thorium Giles Complex gabbro–noritic
intrusions are commonly exposed in the immediate hanging wall of the
Woodroffe Thrust. Deformation and metamorphism during mylonitization did not
significantly alter the original thorium content of the rocks, since the
thorium-bearing phases, such as zircon, allanite, monazite, and apatite, did
not break down during the Petermann Orogeny. Consequently, the original
variation in thorium between the granulite and amphibolite facies rocks was
preserved (Fig. 5). Anomalies in this broad pattern are present locally and
can be attributed to hanging-wall-derived alluvial sediments in the immediate
footwall (lower-thorium anomaly) and to granitoid intrusions of the
Pitjantjatjara Supersuite (higher-thorium anomaly), which are syn- to
post-Musgravian upper amphibolite to granulite facies metamorphism
(Scrimgeour and Close, 1999; Young et al., 2002) (Fig. 5). The contrast
between a lower-thorium hanging wall and a higher-thorium footwall is
well-defined on the airborne thorium map for the central and southern
locations but less evident in the north (Fig. 5). Thorium measurements were
carried out on thin-section chips of felsic gneisses and granitoids via
γ-ray spectrometry, with the measurements run until the error
(2 × standard deviation) was below 10 %. The method is outlined
in detail in Supplement Sect. S1.
Thorium concentrations in felsic gneisses and granitoids from the
hanging wall and footwall of the Woodroffe Thrust, central Musgrave Block.
Thorium concentration in felsic units
Thorium concentrations in felsic gneisses and granitoids (Table 2) have
been compiled from the current study as well as from previous studies of
nearby areas (Camacho, 1997; Young et al., 2002). These data have been
grouped into (1) the central and southern locations and (2) the northern
locations, based on the airborne thorium map, as introduced above.
In group (1), samples unequivocally attributed to either the hanging wall or
footwall of the Woodroffe Thrust were grouped together. This was done on the
basis of their geographical position, with respective samples originating
either from far into the non-mylonitized hanging wall and footwall or from
the lowermost protomylonitic part of the Woodroffe Thrust. In the hanging
wall, thorium concentrations range from 1 to 28 ppm in felsic gneisses and
up to 63 ppm in granitic intrusions (higher-thorium anomalies) but are in
both cases usually lower than 8 ppm. In the footwall, concentrations vary
between 2 and 195 ppm and are typically higher than 10 ppm (Table 2). These
concentrations are in accordance with the regional-scale contrast in thorium
concentrations across the Woodroffe Thrust in the central and southern
locations (Fig. 5), as well as with the results of Lambert and Heier (1968),
who determined concentrations of 2.1 ppm for the granulite facies hanging
wall and 11 ppm for the upper amphibolite facies footwall. Based on the
compilation in Table 2, samples with a thorium concentration < 8 ppm were assigned to the hanging wall and samples with higher
values to the footwall of the Woodroffe Thrust. In general, samples taken
close to the boundary between the mylonites and unsheared rocks of the Fregon
Subdomain (typically the uppermost few tens to one hundred metres) were
investigated. For the majority of samples, the assignment was straightforward
since the inferred hanging wall samples are extremely low in thorium
(< 3 ppm), whereas most inferred footwall samples have values
> 8 ppm. Exceptions are samples SW14-243 and SW13-159, which
have intermediate concentrations of 8 and 6 ppm, respectively. Both samples
were assigned to the footwall since subsequent samples further towards the
hanging wall (SW14-244 and SW13-161) could clearly still be attributed to the
footwall. Alternatively, samples SW14-243 and SW13-159 could reflect
imbrication of the Mulga Park and Fregon subdomains, but this is not
supported by any field observation.
In group (2), thorium concentrations vary between 11 and 49 ppm (Table 2)
but do not allow a clear distinction between samples derived from the
footwall and hanging wall in a manner similar to the central and southern
locations. This result is in accordance with the airborne thorium
concentrations, which also do not indicate a significant jump across the
Woodroffe Thrust in these more northerly locations (Fig. 5).
Determination of boundary between hanging wall and footwall
Measured thorium concentrations indicate that in locations 8 and 11–14
(Fig. 5) the uppermost mylonites of the Woodroffe Thrust developed in the
lower-thorium Fregon Subdomain (Table 2). These results are in agreement with
the proposed identification of hanging wall reworking based on field
appearance. Similar field relationships, such as progressive downwards
mylonitization of units clearly forming part of the hanging wall, also
indicate limited reworking of the Fregon Subdomain at locations 4 and 6
(Fig. 5). However, the lack of a clear contrast in thorium concentrations
between footwall and hanging wall in these locations precludes any
verification of these field observations, as well as any independent
determination of the thickness of hanging-wall-derived mylonites in the
thrust zone. We therefore excluded these northern transects when using
thorium concentration as a proxy to map the original boundary between the
hanging wall and footwall (Fig. 6). It was also not possible to precisely
determine the boundary at location 13 (Fig. 1), due to a large lateral gap
between the last sample assigned to the footwall and the first sample
assigned to the hanging wall. In an attempt to quantitatively calculate the
degree of hanging-wall–footwall reworking into the total mylonite zone, we
applied trigonometry based on the geometrical arrangement sketched in
Fig. 4a. The only modification was that the lower mylonite boundary was now
defined by the newly reconstructed boundary between hanging wall and
footwall. Results are summarized in Table 3. In contrast to the 250 m
proposed by Bell (1978) for the Amata area (western edge of Fig. 1), our
results indicate that only the lowermost 3 m of the Fregon Subdomain was
reworked into the Woodroffe Thrust mylonites at location 8, increasing up to
18–40 m at locations 11–14 (Fig. 1). These values represent 3–6 % of
the entire thickness of the Woodroffe Thrust mylonites (Table 1) at
locations 8, 11, and 14 and up to 13 % at location 12 (Fig. 1). However,
the difference in elevation (p) is not well-defined over short distances
(q) given the limited (30 × 30 m) resolution of the digital
elevation model (ASTER). This can introduce a significant uncertainty into
the calculation of the degree of hanging wall reworking. Nevertheless, our
analysis clearly shows that the majority of mylonites developed in the Mulga
Park Subdomain (footwall) rather than the Fregon Subdomain (hanging wall).
Sample-specific thorium (Th) concentrations (measured by γ-ray spectrometry) plotted across the Woodroffe Thrust (WT) mylonites at
locations 8 and 11–14 (Fig. 1). The original boundary between the
lower-thorium hanging wall and higher-thorium footwall is inferred (red
line).
Presence or absence of fluids during mylonitization
The syn-deformational presence or absence of fluids in the study area is
established from a series of field and thin-section observations. These
include the regional variation in (1) syntectonic quartz veins, (2) the introduction of carbon, and (3) plagioclase stability and breakdown.
Angle of dip (α), elevation difference (p), apparent
thickness (q) and true thickness (T) of the Woodroffe Thrust mylonites
derived from the hanging wall.
Syntectonic quartz veins (Fig. 7a) and associated quartz-rich pegmatite
dykes are uncommon throughout the field area, being generally absent in the
southern locations and only locally present in the northern locations (Fig. 8a). These quartz veins crosscut the mylonitic fabric but were themselves
variably deformed during subsequent shearing, providing direct field
evidence that they were broadly coeval with the Woodroffe Thrust and thus
associated with the Petermann Orogeny. The sense of shear is both top-to-north
and top-to-south, which is contrary to the dominant top-to-north shear sense
associated with the Woodroffe Thrust, but it has also been documented by
Bell and Johnson (1992) in the Amata region (western
edge of Fig. 1). Quartz veins are boudinaged within the mylonitic foliation
and, although deformed, did not preferentially localize strain (Fig. 7a, b).
Field and thin-section images providing evidence for the presence or
absence of fluids during deformation. (a) Quartz veins (encircled)
crosscut the mylonitic foliation of the Woodroffe Thrust showing a sigmoidal
shape consistent with rotation during overall top-to-north shearing. The
quartz vein at the bottom of the picture is boudinaged and did not localize
deformation. Outcrop NW15-264 (coordinates: 131.46883, -25.83040;
location 1 in Fig. 1). (b) Quartz vein adjacent to a sinistral shear
zone that is not reactivated, even though shear zone and quartz vein are
almost parallel to each other. Abbreviation: Sm – mylonitic
foliation. Outcrop SW13-200 (coordinates: 131.73810, -25.99564; location 9
in Fig. 1). (c) Finely dispersed calcite (high birefringence)
between newly formed feldspar grains under crossed polarized light. Thin
section is not oriented. Sample NW14-423A (coordinates: 131.84368,
-26.11423; location 10 in Fig. 1). (d) Plagioclase clast with
muscovite (higher birefringence) and epidote (lower birefringence) inclusions
under crossed polarized light. Thin section is oriented N–S (left–right).
Sample SW14-214A (coordinates: 131.44416, -25.90284; location 5 in Fig. 1).
(e) Plagioclase clast with epidote inclusions under crossed
polarized light. Thin section is oriented N–S (left–right). Sample GW13-415
(coordinates: 131.66256, -25.99928; location 8 in Fig. 1).
(f) Plagioclase clast with kyanite inclusions (greenish needles) and
neo-crystallized garnet under plane-polarized light. Thin section is not
oriented. Sample SW13-167 (coordinates: 131.77475, -26.30845; location 14
in Fig. 1). (g) Inclusion-free plagioclase clast under
plane-polarized light. Thin section is oriented NNE–SSW (left–right).
Sample SW13-318 (coordinates: 132.14311, -25.99142; location 6 in Fig. 1).
Introduction of carbon
Finely dispersed calcite is locally found with very low modal abundance
(typically < 1 %) in the otherwise non-carbonaceous rocks of the
central Musgrave Block (Fig. 7c). Calcite-bearing samples are present
throughout the study area but are generally more common in the north (Fig. 8b). Microstructures indicate that the fine-grained (< 10 µm)
calcite nucleated during shearing (Fig. 7c). Calcium was made available from
dynamic recrystallization of plagioclase to new grains with lower anorthite
content (Wex et al., 2017), but carbon cannot have an
immediately local source because the studied rocks were initially entirely
non-carbonaceous.
Regional variation in the development of (a) syntectonic
quartz veins, (b) the introduction of carbon, and
(c) plagioclase stability and breakdown, each plotted against sample
latitude. The regional temperature gradient (Wex et al., 2017) and the
position of the studied locations (Fig. 1) are indicated.
Plagioclase stability and breakdown
Plagioclase dynamically neo-crystallized in the Woodroffe
Thrust mylonites and associated shear zones (Bell and Johnson, 1989), forming
typical core-and-mantle structures. The composition of the newly
formed grains ranges from
albite/oligoclase in felsic rocks to oligoclase/andesine in mafic rocks (Wex
et al., 2017). Mineral inclusions within plagioclase clasts are common and
allow the distinction of four different types of clasts, termed
microstructures 1 to 4:
Plagioclase studded with abundant inclusions of epidote and muscovite (Fig. 7d). This type is, with one exception, restricted to the northern locations
(Fig. 8c).
Plagioclase containing only epidote inclusions (Fig. 7e), with a modal
abundance far lower than that of epidote + muscovite inclusions of
microstructure 1. This microstructure is restricted to the central locations
(Fig. 8c).
Plagioclase crowded with kyanite needle inclusions (Fig. 7f) (see Supporting
Information B of Wex et al., 2017, for identification
techniques). This microstructure is restricted to the southernmost locations
(Fig. 8c).
Plagioclase free of inclusions (Fig. 7g). This microstructure is found in
almost all locations (Fig. 8c).
Figure 8c shows that the type of inclusions in plagioclase varies in a N–S
direction, i.e. parallel to the tectonic transport direction of the
Woodroffe Thrust. From north to south, inclusions progressively change from
muscovite + epidote (microstructure 1), to epidote (microstructure 2), and
to kyanite (microstructure 3), with inclusion-free plagioclase clasts
(microstructure 4) occurring throughout. There is no apparent variation in
the type of plagioclase inclusions along-strike of the Woodroffe Thrust
(i.e. E–W).
Abundance of hydrous minerals
Pseudotachylytes have been identified as preferred discontinuities for the
nucleation of ductile shear zones and strain localization under mid- to
lower-crustal conditions (Andersen and Austrheim, 2006; Austrheim and
Andersen, 2004; Hawemann et al., 2014, 2018; Lund and Austrheim, 2003;
Menegon et al., 2017; Passchier, 1982; Pennacchioni and Cesare, 1997;
Pittarello et al., 2012; Wex et al., 2014, 2017). In order to study the
control of host rock mineralogy on shear initiation, the modal abundance of
hydrous minerals in deformed and undeformed pseudotachylytes (representative
compilation in Supplement Sect. S3) from felsic footwall and hanging wall
samples was determined by image analysis of backscattered electron (BSE)
images (method outlined in Supplement Sect. S1). The different amounts of
hydrous minerals in the pseudotachylytes should reflect compositional
variations in the host rocks from which they formed. The results, summarized
in Table 4, indicate that matrix mineral assemblages of felsic
pseudotachylytes from the hanging wall and footwall are similar to each
other. These assemblages dominantly comprise
Pl + Kfs + Qz + Bt + Mag + Ilm, with individual samples
also containing Ep, Grt, Cpx, Opx, Ky, Ms, Rt, or Hbl. However, there is a
strong contrast with regard to the modal abundance of hydrous minerals in the
studied pseudotachylytes. Samples have been grouped according to their
latitude coordinates, revealing that (1) the abundance of hydrous minerals
decreases from north to south in both the hanging wall and the footwall and
(2) at similar latitude, the footwall rocks are more hydrous than the hanging
wall rocks. Two samples (SW14-029A and SW14-179) do not fit this regional
trend and these outliers have been excluded in calculating the mean values,
as they mask what are otherwise clear trends.
Modal abundance of hydrous minerals in felsic pseudotachylytes from
the hanging wall and footwall of the Woodroffe Thrust, central Musgrave
Block. Extreme outliers (∗) are not considered. Mineral assemblages
are listed in order of decreasing modal abundance (from left to right) with
hydrous minerals underlined. A representative compilation of pseudotachylytes
is given in Supplement Sect. S3. Errors given are 2 × standard
deviation.
DiscussionPlagioclase breakdown reactions
Within the range of mid- to lower-crustal conditions, as estimated for the
Woodroffe Thrust (Wex et al., 2017), the following plagioclase breakdown
reactions are relevant:
An+Or+H2O=Ms+Ep+Qz
(Kretz, 1963; Ramberg, 1949),
An+Grs+H2O=Ep+Qz
(Kretz, 1963),
An=Grs+Ky+Qz
(Boyd and England, 1961; Hariya and Kennedy, 1968),
An+H2O=Ep+Ky+Qz
(Goldsmith, 1982; Kretz, 1963; Ramberg, 1949).
These reactions proposed in the cited publications are restricted to the
system Ca–K–Al–Si–OH and, in particular, do not consider the presence of
iron. However, iron is certainly necessary to account for the crystallization
of epidote and was potentially derived from relict iron-bearing minerals
(e.g. magnetite, ilmenite, biotite, or garnet). Microstructure 1 is
unequivocally correlated with hydration Reaction (R1). Reaction (R2) is
capable of producing microstructure 2, but garnet has been interpreted to
serve as a calcium sink rather than source during the high-pressure Petermann
Orogeny (Camacho et al., 2009). Alternatively, we propose that the decrease
in anorthite content during plagioclase neo-crystallization
(Wex et al., 2017) provided the necessary source of calcium for producing
microstructure 2. Microstructure 3 is correlated with Reaction (R3).
Reactions (R3) and (R4) represent the high-pressure breakdown of plagioclase
under anhydrous and hydrous conditions, respectively. According to Wayte et
al. (1989), the transition between the two competing reactions occurs at a
water activity of ca. 0.004 for pressure and temperature conditions similar
to those in the more southerly footwall locations. In samples with
microstructure 3, epidote is never observed as a secondary inclusion phase
together with kyanite (Supplement Sect. S4), indicating that Reaction (R3)
always prevailed over Reaction (R4).
Fluid activity
In the southern locations (Fig. 1), plagioclase breakdown by Reaction (R3)
indicates that water was not sufficiently available to drive Reaction (R4) (Goldsmith,
1980, 1982; Wayte et al., 1989). Consequently, Reaction (R3) is a good
indicator of very low water activities (< 0.004; Wayte et al., 1989) and the absence of a free-fluid phase
during deformation, since only very small amounts of water (ca. 20 ppm) are
required for mineral reactions in a solid silicate system (Milke et al., 2013). Similarly, relict
plagioclase clasts without any inclusions (microstructure 4) also indicate
the absence of a free-fluid phase, as the studied rocks were all metastable
with respect to hydration Reactions (R1), (R2), and (R4). Consequently, any of these
breakdown reactions would have rapidly consumed any available free fluid,
since all other reactants were present in the studied samples. Vice versa,
plagioclase breakdown by Reaction (R1) clearly indicates the presence of a free-fluid phase during deformation. Reaction (R2) also involves hydration, as
indicated by the marginal presence of epidote in plagioclase clasts
(microstructure 2). However, free fluids were not sufficiently available to
facilitate Reaction (R1). Hence, we favour the interpretation that
microstructure 2 indicates largely anhydrous conditions, with only very
minor fluid introduction.
Projected schematic cross section through the central Musgrave
Block. The horizontal scale is compressed by a factor of 3.5 with respect to
the vertical scale. The regional temperature gradient is taken from Wex et
al. (2017).
Based on the classification above, the regional availability of a free-fluid
phase during deformation, termed “wet” and “dry”, can be determined from
the mineral inclusions in plagioclase (Fig. 8c) and the corresponding
inferred breakdown reactions (Sect. 8.1). The distinction is purely
qualitative and refers only to whether or not free fluid was sufficiently
available to facilitate the hydrous breakdown Reaction (R1). Consequently,
the studied field area is characterized as dominantly dry (microstructures 2,
3, and 4) with a progression towards locally wet conditions
(microstructure 1) in most of the northernmost exposures (Fig. 9). The
regional variation in (1) the development of syntectonic quartz veins
(Fig. 8a) and (2) the introduction of carbon (Fig. 8b) is consistent in both
cases with this interpretation (Fig. 9). However, within a single location,
individual samples can have a wet microstructure 1 while others still
preserve a dry microstructure 4 (Fig. 8c), indicating that the availability
of fluids varied on a very local scale. Generally dry conditions are also in
agreement with the fact that the metamorphic overprint during the Petermann
Orogeny occurred under lower-grade conditions compared to the earlier
Musgravian Orogeny and thus facilitated water consumption rather than water
release.
Fluid source
There does not seem to be any link between the local distribution of aqueous
and CO2-bearing fluids, since samples that preserve a wet
microstructure 1 are not necessarily calcite-bearing and vice versa (Fig. 8b, c).
Aqueous fluids infiltrating into the footwall of the Woodroffe Thrust were
unlikely to have been derived from the drier hanging wall (Table 4) and
consequently must originate from units within or underlying the current level
of exposure of the footwall. Gneisses and granitoids in the wet northern part
of the study area are clearly interleaved with and juxtaposed onto the basal
units of the Amadeus Basin (Wells et al., 1970), as evident from outcrops of
Dean Quartzite (Forman, 1965; Young et al., 2002)
immediately west of the Kelly Hills klippe. These Neoproterozoic sedimentary
rocks represent an ideal source for aqueous and CO2-bearing fluids
since they were metamorphosed and dehydrated during the Petermann Orogeny
(Wells et al., 1970). However, based on regional-scale geometric
reconstructions, it has been argued that the Woodroffe Thrust and potentially
underlying thrust planes developed in-sequence (Wex et al., 2017). If this is
true, then the Dean Quartzite was only imbricated below the northernmost
studied locations after movement and deformation on the Woodroffe Thrust had
largely ceased and the dynamic microstructures had already been frozen in.
Therefore, we prefer a model in which the aqueous fluids were released from
the granitoids and upper amphibolite facies gneisses within the footwall.
Such a model is in agreement with the fact that the regional trend towards
higher abundance of hydrous minerals in the north parallels the shift towards
wet conditions, as indicated by the plagioclase breakdown reactions (Fig. 9).
An internal source is also consistent with the conclusion that wet conditions
were only present on a very local scale and that wet and dry samples are
often preserved in close proximity. The same is also true of calcite-bearing
and calcite-absent samples. We therefore conclude that the studied field area
was not pervasively infiltrated by externally derived aqueous-rich fluids and
remained a relatively closed system. The source of carbon remains
unexplained, even if the quantities involved are very small. As far as
currently known, units of the footwall Mulga Park Subdomain are
non-carbonaceous, although small amounts of CO2 can be present
within fluid inclusions. Calcite carbon and oxygen (δ13CCal, δ18OCal) and carbon
whole rock (δ13Cwholerock) isotropic
signatures were measured with the aim of providing better constraints on the
source, but the results are inconclusive (see Supplement Sect. S2 for more
details). Values for δ18O are in agreement with calcite
crystallization temperatures of 500–600 ∘C while
δ13CCal values are partially rock-buffered and do
not allow an unequivocal interpretation of the potential source (Supplement
Sect. S2).
Distribution of ductile deformation
The distribution of thorium establishes that the Woodroffe Thrust mylonites
preferentially developed in the Mulga Park rather than the Fregon Subdomain
(Fig. 6). This conclusion is in agreement with our field observations and
with those of Bell and Etheridge (1976), Camacho et al. (1995), and Flottmann
et al. (2004). However, it is evident from our results that in the southern
locations, the lowermost hanging wall was also involved in the mylonitization
process (Table 3). We discuss below the potential processes guiding the
large-scale distribution of ductile deformation in the Woodroffe Thrust.
Hanging wall and footwall reworking
The proportion of the total shear strain accommodated in the narrow mylonitic
to ultramylonitic zones developed in the lowermost hanging wall and uppermost
footwall cannot be determined. However, what could be determined in this
study is the relative thickness of the mylonite zones, using thorium
concentrations to distinguish the original hanging wall and footwall
protoliths. From this it is established that the hanging-wall-derived parts
of the Woodroffe Thrust generally make up < 10 % of the entire
width of the mylonitic zone. This preferential development of a broader
mylonite zone in the Mulga Park Subdomain footwall rather than in the Fregon
Subdomain hanging wall is contrary to the expected simple model of a thrust
or reverse fault system (e.g. Mancktelow, 1985; Passchier, 1984). Such a
model would predict an asymmetric strain profile where the mylonite zone
occurs in the initially “hotter” hanging wall rather than the “colder”
footwall. An inverse distribution of ductile deformation, as a consequence of
asymmetric thermal weakening, would be in agreement with the flower-like
structure model proposed by Camacho and McDougall (2000) for the central
Musgrave Block. Based on the preservation of pre-Petermann K-Ar,
40Ar-39Ar, and Rb-Sr ages in hornblende, muscovite, biotite, and
K-feldspar in the undeformed gneissic country rocks, Camacho and
McDougall (2000) argued that the Fregon Subdomain was rapidly buried and
exhumed in less than 40 Myr. Consequently, these rocks failed to thermally
equilibrate to temperatures above 350 ∘C at a pressure of
∼ 1.2 GPa. With this tectonic model, the hanging wall should have been
at temperatures < 350 ∘C (Camacho and McDougall, 2000) and
thus should have been colder than the footwall
(> 500 ∘C; Wex et al., 2017). In such a model, thermal
weakening could indeed account for preferential mylonitization of the Mulga
Park Subdomain. However, Wex et al. (2017) and Hawemann et al. (2018)
demonstrated that Petermann Orogeny metamorphic assemblages throughout the
study area are directly comparable in dynamically and statically
recrystallized units, arguing that the estimated metamorphic conditions were
ambient and differed little between shear zones and country rock. In
addition, Wex et al. (2017) found that the syntectonic metamorphic conditions
are similar in the hanging wall and footwall at the same locality, arguing
against thermal weakening as a cause of the preferential mylonitization of
the Mulga Park Subdomain (Camacho and McDougall, 2000).
Pseudotachylytes have been identified as preferred nucleation discontinuities
for shearing under mid- to lower-crustal conditions (Andersen and Austrheim,
2006; Austrheim and Andersen, 2004; Hawemann et al., 2014; Lund and
Austrheim, 2003; Menegon et al., 2017; Passchier, 1982; Pennacchioni and
Cesare, 1997; Pittarello et al., 2012; Wex et al., 2014, 2017). In the
Musgrave Block, however, the presence of precursor pseudotachylyte cannot
account for the observed inverse distribution of ductile deformation, since these fault rocks are significantly more abundant in the hanging wall compared to the footwall
(Camacho et al., 1995; Lin et al., 2005).
A potential explanation for the observed inverse gradient of ductile
deformation along the Woodroffe Thrust is a rheological contrast between
hanging wall and footwall assemblages, resulting in different degrees, and
potentially different processes, of water-assisted weakening in quartz and
feldspar (e.g. Fitz Gerald et al., 2006; Griggs, 1967, 1974; Griggs and
Blacic, 1965; Hobbs, 1985; Kronenberg et al., 1990; Mancktelow and
Pennacchioni, 2004; Stünitz et al., 2017; Tullis and Yund, 1989). As
discussed in Sect. 8.3, the rocks in the study area represent a relatively
closed system with respect to the presence or absence of aqueous fluids. The
availability of aqueous fluids is thus directly linked to the abundance of
hydrous minerals in these rocks, which is observed to decrease from north to
south (Fig. 9). Similarly, the contrasting abundance of hydrous minerals in
hanging wall and footwall (Table 4) indicates that there is consistently a
higher potential for generating aqueous fluids in the footwall than in the
hanging wall. If sufficient water was available, this could facilitate a
preferential rheological weakening of the footwall due to new growth of micas
and associated phyllonite development. However, this is not observed because
the majority of the studied rocks are still relatively dry, so that only
slight differences in the abundance of water could facilitate an asymmetric
distribution of ductile deformation as a consequence of water-assisted
weakening. The preferential mylonitization of the Mulga Park relative to the
Fregon Subdomain (Fig. 6) therefore seems to be largely controlled by the
precursor mineralogy established as a result of the
earlier (ca. 1200 Ma) Musgravian Orogeny metamorphism, with peak conditions
of upper amphibolite facies in the footwall but granulite facies in the
hanging wall. This supports the initial hypothesis of Bell and
Etheridge (1976) and Camacho et al. (1995) that a more hydrous footwall
underlying an anhydrous hanging wall can facilitate an inverted gradient of
ductile deformation.
Variation in mylonite thickness
Mylonite thickness in the study area does not significantly vary parallel to
strike (i.e. E–W). However, from south to north, i.e. parallel to the
direction of tectonic transport, the Woodroffe Thrust mylonitic zone
gradually decreases in thickness from over 600 m to less than 100 m
(Table 1). This potentially indicates a 6-fold increase in shear strain (and
therefore average strain rate) within the mylonites, assuming a homogeneous
distribution of strain and constant relative displacement across the
Woodroffe Thrust thickness along the entire ca. 60 km transect. This trend
from south to north is accompanied by a slight decrease in metamorphic
temperature of ca. 100 ∘C (Wex et al., 2017) and the shift from dry
to relatively wet conditions, as reflected in
the increasing abundance of hydrous minerals (Fig. 9). The thickness of
large-scale shear zones is considered to decrease with decreasing temperature
and depth (Platt and Behr, 2011a, b) and decreasing displacement (Hull,
1988). A point in the footwall only enters the shear zone when it passes the
toe of the thrust (that is where the thrust meets the surface). This could
potentially lead to a variation in the finite shear strain experienced in the
footwall. However, the whole of the exposed ca. 60 km N–S section was
formerly at mid- to lower-crustal level (Wex et al., 2017) and thus nowhere
near the thrust toe. There is also currently no evidence for major splays of
the Woodroffe Thrust into the hanging wall. Distributed folding related to
the Peterman Orogeny is observable on the regional scale in the footwall (Wex
et al., 2017), but the amount of shortening involved cannot be quantified. It
follows that there is presently no indication of a significant change in
relative displacement along the studied section and that variation in this
parameter therefore cannot be used to account for the difference in mylonite
thickness. It was argued above that the greater abundance of hydrous minerals
in the footwall Mulga Park Subdomain compared to the hanging wall Fregon
Subdomain, reflecting peak metamorphic conditions of upper amphibolite facies
and granulite facies respectively during the earlier Musgravian Orogeny,
resulted in slightly wetter conditions in the footwall during activity of the
Woodroffe Thrust. This was proposed as an explanation for the broader zone of
mylonites in the footwall. However, the current study also establishes that
the thickness of the mylonitic zone decreases toward to the north and that
this is associated with increasing water activity and decreasing temperature.
Such an observation might reflect the possibility that even a small amount of
fluid was able to weaken the footwall relative to the strong drier hanging
wall, allowing it to preferentially take up deformation. Increasing water
content in the footwall to the north could result in an increase in the
effective rheological contrast between footwall and hanging wall, yielding a
stronger localization toward the interface and a narrower mylonite zone that
extends less into the stronger material. This hypothesis is supported by the
observations in the current paper, but there is not yet
a theoretical basis to provide a full explanation. Decreasing temperature to
the north could also lead to increased localization and decreased shear zone
thickness, both directly (Platt and Behr, 2011a, b) and as a second-order
effect accentuating the rheological contrast as both footwall and hanging
wall become stronger with decreasing temperature.
Conclusions
Field and thin-section observations establish that the rocks of the central
Musgrave Block were predominantly dry during development of the mid-crustal
Woodroffe Thrust during the ca. 560–520 Ma Petermann Orogeny, but with a
progression in the thrust direction towards locally wet conditions in some of
the northernmost exposures. This is indicated by (1) rare occurrence of
syntectonic quartz veins and quartz-rich pegmatites (locally found only in
the north), (2) the metastability of plagioclase in the presence of
K-feldspar, which rarely shows significant alteration via the reaction
An + Or +H2O= Ms + Ep + Qz (more common
towards the north), and (3) preferential high-pressure breakdown of
plagioclase via the reaction An = Grs + Ky + Qz (common in the
southerly exposures), rather than
An +H2O= Ep + Ky + Qz. Aqueous fluids were most
likely derived internally from hydrous minerals within the footwall gneisses
and granitoids, implying that the rocks in the study area were a relatively
closed system.
The thickness of the Woodroffe Thrust mylonites generally decreases with
decreasing metamorphic grade and increasing availability of aqueous fluids.
Atypical of a thrust, ductile deformation is more extensively developed in
the footwall rocks and only marginally involved several tens of metres of
the lowermost hanging wall. The inverse gradient of ductile deformation
cannot be explained by thermal weakening or the distributed presence of
pseudotachylyte (acting as preferred nucleation sites for shearing) but
rather by preferential rheological weakening of the wetter footwall
compared to the drier hanging wall. This reflects the earlier
(Musgravian Orogeny) peak metamorphic conditions (granulite facies in the
hanging wall and upper amphibolite facies in the footwall) and the
contrasting availability of aqueous fluids derived from relict hydrous
minerals in the footwall and hanging wall.
Supplementary data are available in Supplement Sects. S1 to
S4, and further information can be obtained on request from the
corresponding author.
The Supplement related to this article is available online at https://doi.org/10.5194/se-9-859-2018-supplement.
Each of the listed authors took part in at least two of three field seasons,
which formed the basis of this study. AC's previous knowledge
of the Musgrave Ranges and the local communities was essential for the
success of the campaign. NSM and GP developed
the initial idea of the study. SW prepared the manuscript with
contributions from all co-authors.
The authors declare that they have no conflict of interest.
Acknowledgements
We thank the communities of the Anangu Pitjantjatjara Yankunytjatjara Lands
(APY) for granting us access to the Musgrave Ranges. Logistical support from
the Northern Territory Geological Survey (NTGS) of Australia, Basil Tikoff
(Univ. Wisconsin, Madison), and Shane and Alethea Nicolle are gratefully
recognized. We further acknowledge the support of Karsten Kunze from the
Scientific Center for Optical and Electron Microscopy (ScopeM) at the ETH,
Zurich. Jost Eikenberg is thanked for supervising and conducting the thorium
measurements at the Paul Scherrer Institute (PSI) in Villigen, Switzerland.
We further acknowledge the support of Madalina Jaggi, who carried out the
stable isotope analyses at the Geological Institute at ETH, Zurich. This
project was financed by the Swiss National Science Foundation (SNF) grant
200021_146745 awarded to Neil S. Mancktelow, with additional
funding from the University of Padova (BIRD175145/17: The geological record
of deep earthquakes: the association pseudotachylyte-mylonite) awarded to
Giorgio Pennacchioni.
Edited by: Renée Heilbronner
Reviewed by: Florian Fusseis and Kevin Mahan
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