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Tectonophysics 500 (2011) 98–111
Contents lists available at ScienceDirect
Tectonophysics
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Using restored cross sections to evaluate magma emplacement, White Horse
Mountains, Eastern Nevada, U.S.A.
Wayne T. Marko, Aaron S. Yoshinobu ⁎
Department of Geosciences, Texas Tech University, Lubbock TX 79409-1053, USA
a r t i c l e
i n f o
Article history:
Received 23 December 2008
Received in revised form 12 April 2010
Accepted 10 May 2010
Available online 20 May 2010
Keywords:
Pluton emplacement
Contact aureole
Rheology
Magma chamber
Cross section restoration
a b s t r a c t
New field observations and cross section restoration from the Jurassic White Horse pluton–host rock system,
Goshute Range, eastern Nevada, USA, indicate a sequential variation of host rock rheology attending magma
emplacement. The pluton intruded weakly to nondeformed Devonian–Mississippian limestone, argillite and
quartzite at shallow crustal levels (ca. 7 km). The contact aureole is well exposed along the southern, eastern
and northern margin of the intrusive body and is less than 1 km wide. Rocks outside of the aureole are subhorizontal and do not contain a penetrative fabric or are gently folded (interlimb angles N120°) about subvertical axial planes. Within the contact aureole, continuous and discontinuous spaced, axial planar foliations
and harmonic to disharmonic, tight to isoclinal folds wrap around the eastern margin of the pluton. Folds
verge toward and away from the pluton and rim anticlines, synclines, and monoclines with wavelength in
excess of 250 m are preserved along the pluton margin. The spatial proximity of these ductile structures to
the pluton and the apparent increase in intensity of structure development approaching the pluton is
compatible with contraction within the aureole attending pluton emplacement. However, all of the above
structures are truncated by the intrusive contact at various scales. Granodioritic dikes ranging in thickness
from 1 m up to ∼ 10 m emanate from the intrusion and cut host rock structure at high angles and turn to
propagate towards one another, parallel to the pluton margin and host rock anisotropy. Such features are
interpreted to reflect the last stages of diking and brittle deformation that modified the pluton contact after
emplacement-related folding of the carbonate rocks, but before final solidification of the pluton.
Eight serial geologic cross sections were constructed and evaluated to place geometric constraints on the
shape and growth of the White Horse intrusion. Based on line-length restoration of serial cross sections
oriented perpendicular to the pluton contact, the aureole was shortened approximately 54% during
emplacement. Extrapolating this shortening value over the exposed area of the pluton indicates that
approximately 48% of the exposed pluton area may be accounted for by the restoration. Therefore, host rocks
must have been displaced out of the map plane and likely downward based on geometrical constraints
provided by the cross sections. The preferred emplacement model requires in-situ chamber dilation
accommodated by ductile deflection of host rocks followed by fracturing and removal of the aureole rocks via
cauldron subsidence and/or stoping.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The geological record of magma emplacement in arc settings is
inherently incomplete. Only in very shallow crustal conditions or
when thin sills and dikes are observed, can the record of deformation
mechanisms that attend magma emplacement be completely quantified (e.g., Corry, 1988; de Saint-Blanquat et al., 2006). Larger igneous
bodies with elliptical or circular shapes in map view generally have
both concordant and discordant intrusive contacts that vary in
orientation along strike/dip that require multiple deformation
⁎ Corresponding author.
E-mail addresses: wayne.t.marko@ttu.edu (W.T. Marko), aaron.yoshinobu@ttu.edu
(A.S. Yoshinobu).
0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2010.05.001
mechanisms (e.g., ductile flow, plastic creep, brittle failure, etc.) to
accommodate pluton growth through time (e.g., Buddington, 1959;
Barnes et al., 1986; Bateman, 1992; McNulty et al., 1996; Paterson
et al., 1996; Burchardt et al., 2010).
In this paper, structural relations are described within the wellexposed contact aureole of the shallow crustal White Horse quartz
monzodiorite pluton, central Nevada, USA. The host rocks include
Paleozoic carbonate and siliciclastic rocks that show little evidence
for regional penetrative ductile deformation prior to or following
pluton emplacement. Therefore, structures within the contact
aureole may be attributed entirely to processes attending magma
emplacement. Techniques of cross section restoration (‘balancing’;
e.g., Meigs and Burbank, 1997) are employed to evaluate and quantify the amount of bulk shortening within the contact aureole. Serial
geologic cross sections oriented perpendicular to the contact and
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W.T. Marko, A.S. Yoshinobu / Tectonophysics 500 (2011) 98–111
detailed mapping and restoration of the contact geometry demonstrate that space was made for pluton growth by early folding and
ductile flow of the host rocks followed by late, brittle fracturing
along the final pluton contact.
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2. Geologic setting of the White Horse pluton
The White Horse pluton is located in the central-eastern Basin and
Range province of eastern Nevada (Fig. 1A). This area is host to over
Fig. 1. A: Simplified map of Jurassic intrusions and regional structure in the Basin and Range Province (modified from Elison, 1995, and Miller and Allmendinger, 1991). B: Simplified
geologic map of the White Horse pluton and host rocks modified from Silberling and Nichols (2002).
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fifteen plutons ranging from Jurassic to Cretaceous in age that intruded a
thick series of marine platform and shelf strata now exposed in the backarc region of northeast Nevada. The eastern part of the White Horse
pluton and contact aureole are well exposed over an area of approximately 16 km2 with approximately 450 m of vertical topographic
relief. An unknown area of the pluton is covered by Quaternary alluvium
to the west (Fig. 1B). The pluton contact is exposed over a linear distance
of approximately 7 km (at 1:12,000 scale) and has a jagged trace in plan
and profile views with several apophyses intruding parallel and
perpendicular to host rock anisotropy (Fig. 2).
2.1. Regional deformation
In order to differentiate between emplacement-related and
regional-related deformation, we first summarize the regional
structure within the field area. Mesozoic structures consist of northtrending gentle regional folds (Ketner et al., 1998), normal faults
(Miller and Allmendinger, 1991) and low to moderate angle faults
termed “attenuation faults” by Silberling and Nichols (2002). The
“attenuation faults” (normal faults on Fig. 2) were responsible for
stratigraphic thinning of some of the Mississippian strata in the region
and are generally interpreted to be Jurassic in age. Many of these
structures are cut by or modified by Late Jurassic intrusions and may
be as old as Early Jurassic (Silberling and Nichols, 2002). Similar
structures have developed or been reactivated during Cenozoic extension complicating the interpretation of regional structural evolution (Ketner et al., 1998).
The initiation of “attenuation faults” (i.e., low angle normal faults)
and emplacement of the White Horse pluton are the earliest tectonic
events in the region (Silberling and Nichols, 2002). Silberling and
Fig. 2. New simplified geologic map of the White Horse pluton, eastern Nevada, after Marko (2004). (This figure is spread over two pages in manuscript and publication form).
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101
}
Fig. 2 (continued).
Nichols (2002) suggest that contact metamorphosed argillite of the
Chainman Shale and pluton-related dikes in the hanging wall of the
structurally highest normal fault constrain extensional fault development as pre- to syn-emplacement. This interpretation is in agreement
with our observation that the normal faults along the outer, southeastern margin of the White Horse pluton structural aureole have been
rotated into their current, steeper orientations during pluton construction. The role of the “attenuation faults” during emplacement of the
White Horse pluton will be further discussed below.
Cenozoic deformation included reactivation of the Ferguson
Mountain detachment fault to the north (Ketner et al., 1998) and
development of a number of high-angle normal faults, some of which
intersect the aureole. These younger structures are too far from the
White Horse contact aureole (Fig. 1) or are too weakly developed to
have altered emplacement-related structures in the aureole.
2.2. White Horse pluton–host rock system
The pluton consists of quartz monzodiorite with lesser amounts of
porphyritic granodiorite near the margins and a fine-grain muscovite
quartz monzonite facies in the westernmost exposures. Aplitic granite
to granodiorite dikes intrude all three phases of the pluton (Messin,
1973). Pluton-related granodioritic dikes intrude the host rock outside the aureole. A K–Ar hornblende date of 160 Ma (Miller and
Hoisch, 1995) and biotite date of 157 ± 6 Ma (Messin, 1973) indicate a
Middle to Late Jurassic age. Aluminum-in-hornblende barometry and
corrected stratigraphic reconstructions of Paleozoic sedimentary
rocks in the region indicate emplacement depth at b7.5 ± 1.9 km
and 7.1 ± 2.0 km, respectively (Miller and Hoisch, 1995).
In general, the pluton is metaluminous with an A/CNK ratio of 0.98.
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Initial Sr /Sr and εnd values are 0.7072 and −5.5, respectively (Miller
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and Hoisch, 1995). The peak regional temperatures based on
conodont metamorphic index range from 300 to 400 ° C (Welsh,
1994; Silberling and Nichols, 2002). Plutonic samples have hypidiomorphic texture and lack evidence for significant subsolidus or
hypersolidus deformation although quartz grains commonly display
undulose extinction.
Magmatic fabrics such as igneous layering and foliations are only
locally observed in outcrop. Magmatic foliations are defined by
aligned subhedral to euhedral orthoclase and plagioclase phenocrysts
and do not show a consistent trend at the map scale (Fig. 3A).
Magmatic lineations were not observed. Minimal evidence for
dislocation creep included deformation twins in feldspar and minor
subgrain development in quartz is consistent with formation of the
foliation above the solidus (e.g., Paterson et al., 1989). Quartz biotite
microdiorite enclaves are common within the coarser grained phases
of the pluton and range from 2 cm to 1 m in length and have an
ellipsoidal to oblate shape. No penetrative subsolidus fabric was
observed in the pluton.
Fig. 3. Lower hemisphere projections of various structural elements in the White Horse pluton–host rock system; A) poles to magmatic foliations as measured in outcrop; B) poles to
bedding outside of the contact aureole; C) poles to bedding in the northern structural domain; D) poles to bedding in the northeastern structural domain; E) poles to bedding in the
southeast structural domain; F) lineations and fold axes within the entire contact aureole.
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Host rock xenoliths are rarely observed within the White Horse
pluton with the exception of the large potentially unconnected block
along the northern contact (Fig. 2). This marble block has a hornfelsic
texture and does not preserved any sedimentary or tectonic
structures or fabrics. Without these structures it is difficult to establish
whether the block is attached to the wall or has been rotated in the
pluton.
2.3. Host rocks and contact aureole
The pluton intrudes Devonian limestone, dolostone and quartzite
of the Guilmette Formation and argillites of the Mississippian
Chainman Shale. Within the contact aureole marbles consist of fine
to coarse crystals of (2–3 mm) of calcite and dolomite. Dolomite and
calcite interlayering is common and may range in scale from several
millimeters to several meters in thickness. Quartzite is commonly
fine-grained, weathers brown, tan or white, and is exposed over a
range of thicknesses from 10 cm to 1.5 m. Tabular cross bedding is
commonly preserved but is generally cryptic. Individual quartzite
marker units may occur as single beds or pairs and may be mapped
over distances of greater than 2.5 km.
Limestone and dolostone are commonly light gray to blue gray,
massive, with bedding up to 1 m thick. Textures range from mudstone
to grainstone. Fossiliferous beds within limestones are common and
consist of recrystallized and weakly deformed amphipora, gastropods,
ostrocods, bivalves and various shell fragments. Bedding has been
weakly folded into a north–south trending regional open anticline,
which may be a continuation of the regional scale anticline mapped by
Ketner et al. (1998). Outside the contact aureole, host rocks are subhorizontal or are weakly deformed into low amplitude folds with
interlimb angles greater than 120° (Fig. 3B).
The contact aureole extends from the pluton contact to the outer
limit of the exposure of foliated, metamorphosed, and isoclinally to
openly folded host rocks with interlimb angles b120°. The aureole
ranges in thickness from 100 m to 800 m in plan view (Fig. 2). The
aureole is narrow along the southern and southeastern margin where
contact metamorphosed argillites of the Chainman Shale and the
White Horse Pass limestone are juxtaposed against the Guilmette Fm.
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along a normal fault (Fig. 2, adjacent to cross section H–H′). These
units are not internally deformed in the vicinity of the pluton, but
were rotated about a horizontal axis into their current position during
emplacement based on regional considerations (Silberling and
Nichols, 2002) and the style of deformation observed in the aureole
in this study (e.g., Figs. 2 and 5), and thus are included within the
approximate aureole boundary. Silberling and Nichols (2002) also
noted that the Chainman Shale argillite has a hornfels texture, thus
the thermal aureole must extend into the hanging wall of their
“regional attenuation faults” (Fig. 2). The widest exposure of the
contact aureole is preserved along the north and east margins of the
pluton (Fig. 2).
Contact metamorphism within the aureole resulted in olivine,
calcium (Ca) pyroxene, tremolite, and talc porphyroblast growth and
grain coarsening (Fig. 2; Marko, 2004). Antigorite is retrograde after
olivine. These minerals are restricted to the dolomite and calcite
marbles of the Guilmette Formation. Porphyroblasts range from
anhedral to euhedral and lack a shape-preferred orientation. Olivine
occurs as recrystallized and fractured aggregates occurring in places in
an unusual tabular morphology up to 2.5 mm in length; however,
individual grains may be as small as 0.01 mm in diameter. Grains tend
to have uniform extinction and no shape-preferred orientation. Grain
boundaries appear cuspate and possibly embayed in some grains. Talc
and tremolite are present just outside of the contact aureole in finegrained undeformed host rocks and demarcate the regional metamorphic grade. Contact metamorphism is less well defined along the
southeastern and southern contact because the outcrop exposure is
poor and hornfelsic textures are predominant.
Phase stability equilibria were calculated at 2 kbars, based on
an emplacement depth of ∼7 km, using GeO-Calc (Brown et al., 1988).
Peak metamorphic conditions are constrained by the reaction
diopside +dolomite = forsterite + calcite + CO2 which is stable between 590° and 630 °C over a wide range of CO2 mole fractions
(Brown et al., 1988). The calcite + antigorite (relict after forsterite) +
dolomite assemblage is found as far as 400 m from the contact
suggesting that peak thermal conditions likely extended greater than
400 m from the pluton contact. Peak metamorphic conditions at
the outer aureole are likely greater than 400 °C as constrained by the
Fig. 4. Photomosaic view to the east and geologic interpretation displaying discordant relationship between mesoscale folds in the contact aureole and the intrusive pluton contact.
Location of photomsaic on Fig. 2. Sub-horizontal beds in upper left of photo display regional dip. Note the rim monocline–syncline–anticline triplet that is truncated by the pluton
contact.
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calcite + tremolite assemblage. These values are higher than the peak
regional temperature estimates of 300 °C based on the conodont
metamorphic index from samples near White Horse Pass (Silberling
and Nichols, 2002). However, Welsh (1994) reported peak temperature
estimates near Furgeson Mountain north of White Horse Pass as high as
400 °C.
2.4. Deformation in the contact aureole
Rocks within the contact aureole exhibit folds with axial planar
foliations that are broadly parallel in map trace with the pluton
contact (Fig. 2). Isoclinal to open folds with sub-horizontal fold axes
and wavelengths of up to several hundred meters wrap around the
pluton and are continuous for 6 or 7 km along the entire exposed
length of the contact aureole. Fold interlimb angles and axial planar
foliation intensity decrease dramatically away from the pluton
contact, further delineating the outer edge of the contact aureole
(Fig. 4). Eight serial geologic cross sections have been constructed
through the contact aureole and transect the pluton/host rock contact
(Fig. 5). Cross section form lines were approximated from measurements of bedding, quartzite marker beds, and formation boundaries
where observed.
Three structural domains may be defined based on the style and
nature of folding in the contact aureole and the relationship between
the axial surfaces and pluton contact. Folds in the northern domain
(Figs. 3C and 5, cross sections A–A′, B–B′, and C–C′) exhibit tight to
isoclinal interlimb angles and have axial planes that dip moderately
(30°–40°) away from the pluton. Section B–B′ shows an overturned
rim syncline adjacent to the pluton contact that is continuous with a
monocline in the outer aureole. The outer sub-horizontal limb of the
Fig. 5. Serial geologic cross sections through the contact aureole of the White Horse pluton. Map symbols as in Fig. 2.
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monocline likely reflects the regional stratigraphic dip (see also
Fig. 4). Fold wavelengths range from approximately 170 to 260 m and
amplitudes range from 70 to 130 m. The axial planes of these folds are
within 15° to sub-parallel to the outward-dipping pluton contact in
this structural domain.
The eastern domain (Figs. 3D and 5, cross sections D–D′, E–E′) is
dominated by smaller wavelength/amplitude, tight to locally isoclinal
folds. As many as six distinct mesoscale folds have been identified and
mapped within the 0.5 km wide contact aureole along the eastern
margin of the pluton (Fig. 2). This domain also displays a transition in
contact dip from outward to inward as one progresses to the south
along the contact (see below).
In contrast, folds in the southeast and southern domain (Figs. 3E
and 5, cross sections F–F′, G–G′, H–H′) are most commonly isoclinal
adjacent to the contact but contain axial planes that dip both toward
and away from the pluton contact. Here, fold wavelengths range from
20 to 90 m and amplitudes range from 30 to 40 m.
The dominant outcrop-scale metamorphic fabric within the
structural aureole is a discontinuous, spaced foliation oriented parallel
to subparallel with the axial planes of map-scale tight to isoclinal
folds. This axial planar foliation is defined by coarse 1–3 mm flattened
carbonate aggregates. A fine, continuous foliation wraps around white
coarse-grained carbonate aggregates up to 4 cm in length. Minor
parasitic “s”- and “z”-folds occur on the scale of 10 cm to 2 m. These
folds exhibit shallowly plunging axial trends parallel to the pluton–
host rock contact and are commonly pluton vergent (Fig. 3F).
A number of significant observations may be extracted from the
serial cross sections and geologic map. First, rim anticlines, monoclines and synclines occur along the margin of the pluton. Second, the
majority of map-scale folds verge toward the pluton. Third, limbs of
folds are discordant to the pluton margin along sharp intrusive
contacts (Figs. 2, 4, and 5).
2.5. Pluton–host rock contact in combined serial cross sections
For this study, the pluton–host rock contact was mapped in detail
at 1:6000 to evaluate changes in orientation and angular relationships
between the contact and structures in the host rocks. Fig. 6 shows the
projected orientation of the contact geometry along its entire exposed
length. The contact everywhere observed is intrusive, except for a
small segment along the southern end where a Tertiary fault cuts the
contact, and is exposed over a structural elevation of 450 m and
contains numerous jogs, apophyses and curves. These deviations from
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planarity occur over wavelengths ranging from meters to kilometers.
Along the north and northeastern margin, the contact changes from 80°
to ∼30° outward. In this region, the contact truncates a syncline fold axis
250 m west of cross section A–A′. Between cross sections A–A′ and C–C′
the contact shape is more curvi-planar and the outcrop trace tends to
follow topography. Here, a three-point solution yields a dip of ∼30° in
cross sections B–B′ and C–C′. Between cross sections C–C′ and D–D′ the
contact dips 21° to the east. Detailed mapping shown in Fig. 7
demonstrates how the contact maintains a curvi-planar shape and
steps left (west) exposing 0.75 km of strike-length of an anticline that
has been truncated at both ends by abrupt changes in the pluton host
rock contact orientation. The dip of the contact near D–D′ was calculated
with a three-point solution and dips 45° to the southwest (Fig. 7. In
contrast, the dip of the orientation measured 250 m to the north at
section C–C′ is 34° to the northeast. The pluton host rock contact below
2100 m in section D–D′ was projected into the section from C–C′
whereas the upper contact was measured with a three-point problem.
Between sections E–E′ and G–G′ the contact dips moderately to steeply
to the southeast. At section H–H′ the contact dips ∼14° to the northwest.
The trace of the contact between sections E–E′ and F–F′ (Figs. 2
and 5) displays apophyses, which truncate host rock foliation (Figs. 2
and 5). The tips of these apophyses have intruded parallel to the host
rock fabric and point toward each other. The outer contact of the
southern apophyse dips 46° to the southeast. The map trace of pluton
host rock contact between cross section lines F–F′ and G–G′ is
straight and cuts across topography. The calculated dip is 88° to the
southeast and is similar to bedding and foliation orientations in the
area. Between cross sections G–G′ and H–H′ the contact trace is curviplanar and truncates a contact-parallel syncline. Two igneous
apophyses extend into the host rocks. The apophyse 200 m north of
cross section H–H′ (Fig. 2) cuts across an anticline and “V's” down a
small drainage. The calculated contact dip at this location is 14° to
southeast. The contact dip is similar to the dip of local host rock
bedding. Near cross section H–H′ the trace of the contact is curved
and outlines a small embayment of marble within the intrusion. The
contact dips ∼ 14° to the northwest. The apparent embayment of the
contact is the result of the shallow inclination of the pluton host rock
contact and the structural position of plutonic rocks above marble
host rocks.
The variable contact geometry and the presence of local subhorizontal contacts might suggest that the roof (∼ elevation of 2050 m
cross sections C–C′ and D–D′), walls, and floor (∼ elevation 1930 m
cross section H–H′) of the pluton are exposed. However, the vertical
Fig. 6. Oblique perspective view of the pluton contact as constrained from geologic mapping and serial cross section development. View is to the east. Note the complex threedimensional geometry of the contact including the N 300° change in orientation of the contact between cross sections C–C′ and D–D′. 1900 m elevation contour interval is shown on
the contact for reference. Inset map displays oblique perspective view and cross sections utilized for the reconstruction. See text for discussion.
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Fig. 7. Detailed geologic map (after Marko, 2004) of a portion of the northeastern contact of the White Horse pluton (see Fig. 2 for location). White arrows point to regions where the
traces of folds are cut by the intrusion. See Fig. 4 for a photomosaic of the cross-cutting nature of the pluton and the folds depicted in this figure.
thickness between the potential roof and floor elements is 120 m and
this thickness is inconsistent with the ∼ 350 m of exposed structural
relief of the pluton. Therefore, sub-horizontal contacts are likely jogs
in the chamber wall.
3. Results and discussion
Because of the lack of appreciable ductile deformation outside
of the structural aureole it is possible to evaluate emplacement-
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related deformation and test models for pluton construction in the
shallow crust. Below we describe two partially restored geologic
cross sections to estimate the minimum amount of bulk shortening
that has accrued within the structural aureole during magma emplacement (Fig. 8). We then explore the nature of contact relations
between the pluton and host rocks, and conclude with an analysis
of ‘rim folds’ in contact aureoles.
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3.1. Structural restoration of ductile deformation
Longitudinal strain has been calculated for the northern contact
aureole. The exposed and interpreted continuity of meta-quartzite
marker beds at depth was used for line-length restoration of folding in
cross sections A–A′ and C–C′ (Fig. 8). This analysis assumes plane strain
in the cross sectional area and does not account for heterogeneous strain
Fig. 8. Restored geologic cross sections A–A′ and C–C′ showing method of longitudinal strain calculation.
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and/or volume loss in the cross section plane. Because quartzite marker
beds are continuous over the map area, we contend that the longitudinal
strain estimates provide a viable estimate of the exposed contact aureole
bulk shortening value (e.g., Paterson and Fowler, 1993). Because markers
are truncated along the igneous contact, these estimates are minimum
values. The current geometry of these markers was reconstructed from
both the mapped exposures and the projection of the beds into cross
section B–B′ (Figs. 2 and 5). A regional pin line was placed outside of the
aureole and the extent of the aureole was estimated from fold geometry.
The horizontal length of the marker bed was measured within the
aureole and then compared with the restored horizontal line length form
the local pin line place at the outer limit of the contact aureole. Cross
section restoration of quartzite marker beds in sections A-A′ and C-C′
indicate as much as ∼54% shortening in the northern aureole.
The aureole bulk shortening value was then extrapolated to
calculate a plan view aureole area restoration (Fig. 9). The plan
Fig. 9. Pluton area restoration utilizing bulk shortening estimates from retrodeformed cross sections. This 2-D plan view restoration assumes ∼ 54% shortening perpendicular to the
pluton contact. “Undeforming” the serial geologic sections from Fig. 3 leaves 52% of the space currently underlain by the pluton unaccounted.
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view thickness of the aureole was measured from Fig. 2. As a first
approximation, 54% shortening derived from the line-length restoration of cross sections A–A′ and C–C′ was assumed throughout
the exposed aureole (Figs. 8 and 9). Following this methodology,
restoration of the 54% shortening in the exposed aureole accounts
for ∼ 48% of the pluton map area (Fig. 9). Therefore, approximately
half of the host rock area now occupied by the pluton must have
been transported vertically out of the map plane or currently
reside under alluvium to the west. This calculation assumes that
the aureole exposed in plan view is complete and that shortening
in the entire aureole was 54%. The southern part of the contact
aureole may have been thinned by a reactivated, younger attenuation fault and may therefore yield a minimum value for
restoration area. In addition, the fold geometry in the southernmost exposures of the contact aureole would suggest much lesser
amounts of total shortening in the aureole. Therefore, the 54%
shortening of the entire aureole may not fit for this portion of the
map area.
A general conclusion of the plan view restoration is that
shortening within the exposed pluton host rocks does not account
for the total exposed portion of the pluton. It is conceivable that much
larger values of bulk shortening in the unexposed regions of the
pluton–aureole system may have accommodated the area (and
volume) that is apparently missing. However, there is no measurable
increase in bulk shortening or strain approaching the unexposed
western contact with the host rocks (e.g., compare cross sections in
Fig. 5). These observations and interpretations are consistent with
the assumption that the bulk shortening values do not vary
significantly between exposed and unexposed regions of the host
rocks. Therefore, other emplacement mechanisms must have been
operative to accommodate magma emplacement and host rock
displacement.
3.2. Late-magmatic contact modification
A number of observations presented here lead to the conclusion
that the contact geometry has been modified after magma emplacement and syn-emplacement folding, but before solidification of the
pluton. First, bedding, axial planar foliations and other host rock
structures are truncated along margins that vary by greater than 90°.
If folding occurred after initial sharp contact formation, structures in
the host rocks adjacent to the contact should be transposed into subparallel orientations relative to the contact. Second, dikes emanating
from the pluton cut across and parallel to host rock anisotropy at
various scales (e.g., between cross sections E–E′ and F–F′, Fig. 2).
These contact relations indicate that magma was present well after
the formation of these anisotropies. For example, along the eastern
margin of the pluton two large map-scale apophyses and many small
dikes intrude the host rocks preserving “flaps” of marble and
dolostone 100s of meters long within the pluton (Fig. 2). Third,
contacts are generally sharp and discordant at the outcrop scale,
implying that material has been removed from the map plane (e.g.,
Figs. 2, 4, and 5).
3.3. Preferred pluton growth model
Given the observations and interpretations above, it is possible
to account for over half of the exposed area of the pluton with
ductile shortening in the wall rocks and vertical translation of the
host rocks by a brittle deformation mechanism. We view ductile
shortening of the host rocks to be the primary mechanisms of
pluton growth, followed by some type of piston-style floor downdrop (e.g., stoping or cauldron subsidence; Daly, 1903; Clough
et al., 1909; Clarke et al., 1998; Zak and Paterson, 2006) that
produced discordant pluton–host rock contact relations.
109
Absolute ordering of emplacement processes from initial formation of a magma chamber to final solidification of the pluton is
uncertain. Because of the impossibility of completely restoring the
host rocks to their pre-emplacement geometry, we develop a stepwise model beginning from final solidification backwards in time. We
note the increasing uncertainty and ambiguity in the early (i.e.,
oldest) time slices in this model. We suggest that brittle failure was
the final deformation mechanism that accommodated pluton growth
and resulted in the discordant intrusive contact observed. Prior to this
brittle event, pluton growth occurred by host rock ductile flow as
magma was being emplaced. Strain within the contact aureole
increased proximal to the contact (Fig. 10). However, rates of ductile
flow within the host rocks could not keep pace with rates of magma
emplacement into the growing intrusion. Therefore, the host rocks
failed brittlely at elevated strain rates. Host rocks were subsequently
removed, translating the hypothetical map view contact outward and
host rock blocks down (Fig. 10). The model presented here is analogous to pluton construction via ‘ballooning’ (e.g., Sylvester et al.,
1978; Holder, 1979; Brun et al., 1990; Tikoff et al., 1999; Johnson et al.,
2001). However, in our model ductile flow attending ballooning is
superseded by brittle failure, while the pluton still contains magma.
Given the lack of observed host rock xenoliths within the pluton, we
are lead to the somewhat problematic conclusion that the magma was
sufficiently melt-rich to allow blocks to migrate downward.
Diapirism, the buoyant rise of large volumes of magma by viscous
host rock deformation, has been proposed to explain the geometry of
intrusion of large, circular to elliptical plutons (Paterson and Vernon,
1995). However, diapirism has come under scrutiny because models
of hot stokes diapirs are thermally limited in their ascent distances
(Marsh, 1982). More recent treatments of crustal diapirism as
controlled by power law creep rather than Newtonian flow (compare
Marsh, 1982 with Weinberg and Padladchikov, 1994) have met
significant resistance primarily because predicted field observations
(high strain aureoles, concordant rim synclines) are not widespread
(Petford, 1996). Following theoretical work of Rubin (1993), Miller
and Paterson (1999) suggested that the host rock deformation
associated with diapirism need not be entirely viscous. Instead, they
suggested the term ‘visco-elastic diapir’ to explain the ascent of large
batches of magma through a crust that deforms by simultaneous
plastic and brittle deformation mechanisms during magma migration.
We suggest that fluctuating strain rates during magma emplacement
into the growing White Horse pluton is a visco-elastic response due to
emplacement rates outpacing ductile creep rates, leading to strain
hardening and finally brittle failure.
3.4. Implications rim folds around plutons
Several rim monocline/anticline relationships have been recognized along the margins of plutons in the Great Basin region (e.g.
Allmendinger and Jordan, 1984, and Allmendinger and Miller, 1991).
Subsequent workers (Zamundio and Atkinson, 1995; Glazner and
Miller, 1997; Miller and Bedford, 1999) have called on gravitational
forces to cause these structures to form primarily by ‘pluton sinking’
upon density increases that occur at crystallization. However,
structures in the White Horse aureole that might support such a
model such as steep, inward-dipping pluton host rock contacts,
steeply plunging, inward-dipping lineations, and monoclinic fabrics
produced by downward flow of host rocks and pluton are not
observed. Instead, the tight upright fold geometry (Figs. 4 and 5) is
more consistent with broadly horizontal shortening of the contact
aureole. We suggest that rim folds may occur due to pluton
construction and that their final geometry, especially where discordantly cut by intrusive igneous contacts (e.g., Fig. 7; Glazner and
Miller, 1997), may have more to do with late contact modifications
while the pluton is still partially molten than with kinematics
attending magma solidification and sinking.
Author's personal copy
110
W.T. Marko, A.S. Yoshinobu / Tectonophysics 500 (2011) 98–111
Fig. 10. Time integrated view of magma being emplaced into the growing White Horse pluton and the resulting shortening in the aureole followed by brittle fracturing and removal
of host rocks out of map plane. Multiple pulses of magma have intruded the growing pluton. The top figure is an oblique perspective view based on the geologic map in Fig. 2 with the
Quaternary sediment removed. T1–T5 represent arbitrary time steps in the diagram from earliest to latest. Subsurface constraints for the ‘present day’ configuration are from cross
section lines A–A′ and H–H′ in Fig. 5. Dilation of the chamber occurs via shortening of the walls forming rim folds. During and subsequent to ductile shortening the wall rocks are
discordantly removed along intrusive contacts.
4. Conclusions
Growth of the White Horse pluton may be documented by restoring
serial cross sections and evaluating bulk shortening in the contact
aureole. Structural observations yield the following conclusions:
1. Folding and ductile deflection of host rocks are solely attributed to
the emplacement of magma.
2. Fold axes and limbs at all scales are truncated by the intrusive
contact. Therefore, folding predated final contact modification.
3. Serial cross section restoration indicates that the aureole was
shortened by approximately 54%.
4. Extrapolation of the shortening values over the entire plan view of
the exposed contact aureole indicates that approximately 48% of
the pluton area was generated by host rock shortening. Other host
rock displacement mechanisms (e.g., stoping, cauldron subsidence,
chamber floor down-drop, and chamber roof uplift) must account
for the rest.
5. Rim folds including synclines, anticlines, and monoclines may not
be a function of kinematics associated with pluton sinking.
6. The rheological transition from early ductile flow to late brittle
failure within the host rocks was accommodated while the pluton
was still partially molten. We conclude that such relations may
occur where early ductile flow cannot keep pace with magma flux
into a growing pluton. Therefore, brittle failure of the chamber
walls may dramatically increase aureole strain rates to accommodate magma emplacement.
Acknowledgments
This work was funded by NSF Grant EAR-0106557 to Yoshinobu.
We thank Weston and Miles Yoshinobu, Brian Cornwell, Celeste
Thomson, and Don and Nat Wielenga for field assistance in the Great
Basin. Discussions about pluton emplacement over the years with
Scott Paterson, Ken Fowler, Bob Miller, Cal Barnes, Brendan McNulty,
Øystein Nordgulen, Othmar Tobisch, John Bartley, Allen Glazner, Greg
Davis, Geoff Pignotta and Greg Dumond, and the numerous fine
geologic maps produced by many USGS field geologists during the last
century have helped shape our questions, observational skills,
interpretations, and biases. We are grateful to all who have contributed to this knowledge. We thank David Miller for sharing his
deep knowledge of the geology and tectonics of the Great Basin. ASY
Author's personal copy
W.T. Marko, A.S. Yoshinobu / Tectonophysics 500 (2011) 98–111
thanks Andrew Meigs for introducing him to balancing cross sections
over the copy machine at USC in 1995.
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