Article
Volume 13, Number 10
19 October 2012
Q10013, doi:10.1029/2012GC004341
ISSN: 1525-2027
Constraining volcanic inflation at Three Sisters Volcanic Field
in Oregon, USA, through microgravity
and deformation modeling
Jeffrey Zurek and Glyn William-Jones
Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British
Columbia V5A 1S6, Canada (jmz3@sfu.ca)
Dan Johnson
Deceased 4 October 2005
Al Eggers
Department of Geology, University of Puget Sound, 1500 N. Warner Street, Tacoma, Washington
98416-5000, USA
[1] Microgravity data were collected between 2002 and 2009 at the Three Sisters Volcanic Complex,
Oregon, to investigate the causes of an ongoing deformation event west of South Sister volcano. Three
different conceptual models have been proposed as the causal mechanism for the deformation event:
(1) hydraulic uplift due to continual injection of magma at depth, (2) pressurization of hydrothermal systems
and (3) viscoelastic response to an initial pressurization at depth. The gravitational effect of continual magma
injection was modeled to be 20 to 33 mGal at the center of the deformation field with volumes based on previous deformation studies. The gravity time series, however, did not detect a mass increase suggesting that a
viscoelactic response of the crust is the most likely cause for the deformation from 2002 to 2009. The crust,
deeper than 3 km, in the Three Sisters region was modeled as a Maxwell viscoelastic material and the results
suggest a dynamic viscosity between 1018 to 5 1019 Pa s. This low crustal viscosity suggests that magma
emplacement or stall depth is controlled by density and not the brittle ductile transition zone. Furthermore,
these crustal properties and the observed geochemical composition gaps at Three Sisters can be best
explained by different melt sources and limited magma mixing rather than fractional crystallization. More
generally, low intrusion rates, low crustal viscosity, and multiple melt sources could also explain the whole
rock compositional gaps observed at other arc volcanoes.
Components: 9300 words, 7 figures, 2 tables.
Keywords: Three Sisters Volcanic Field; gravity; viscoelastic; volcanology.
Index Terms: 1217 Geodesy and Gravity: Time variable gravity (7223, 7230); 8419 Volcanology: Volcano monitoring
(4302, 7280); 8439 Volcanology: Physics and chemistry of magma bodies.
Received 11 July 2012; Revised 18 September 2012; Accepted 24 September 2012; Published 19 October 2012.
Zurek, J., G. William-Jones, D. Johnson, and A. Eggers (2012), Constraining volcanic inflation at Three Sisters Volcanic Field
in Oregon, USA, through microgravity and deformation modeling, Geochem. Geophys. Geosyst., 13, Q10013, doi:10.1029/
2012GC004341.
©2012. American Geophysical Union. All Rights Reserved.
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1. Introduction
[2] Large-scale deformation events are common at
active volcanoes and are most often detected using
seismic or deformation techniques [e.g., Dzurisin
et al., 1990]. While these methods can provide
information about the volume and shape of the
deformation source, they do not constrain the change
or redistribution of mass at depth. Microgravity surveys, when used in conjunction with deformation
data, can provide constraints on the mass flux at
depth and source density [e.g., Berrino et al., 1992;
Rymer, 1996; Battaglia et al., 2003].
[3] Microgravity has been utilized on numerous volcanic systems, including Yellowstone (USA) and
Campi Flegei (Italy) calderas to constrain and investigate the properties of deformation sources. At both
calderas, changes in the associated hydrothermal
systems have been postulated by several authors
[e.g., Dzurisin et al., 1990, 1994; Berrino, 1994;
Bonafede and Mazzanti, 1998; Orsi et al., 1999;
Gottsmann et al., 2006] as a possible source of the
observed deformation and gravity change. Microgravity provides the possibility to distinguish between
a magmatic and a hydrothermal source, since it can
determine the density of the intruding fluid. Natural
systems are usually complex and can have multiple
deformation sources [e.g., Dvorak and Berrino, 1991;
Trasatti et al., 2011]. For example, Gottsmann et al.
[2006] show that it is more realistic to model the
deformation and gravity data at Campi Flegei as a
combination of magmatic and hydrothermal sources.
When determining the nature of a deformation event,
it is also important to consider the possibility that the
Earth’s crust may be deforming viscoelastically; an
elastic deformation model can result in unrealistic
overpressures needed to reproduce the observed
uplift [e.g., Berrino et al., 1984]. Furthermore, volcanic areas generally consist, at least in part, of
incoherent material produced from eruptions and a
high crustal heat flow [e.g., Bonafede et al., 1986],
which produces a lower effective viscosity for the
crust and thus requires the consideration of its
inelastic properties.
[4] Integrating deformation and microgravity has
become the standard approach to determine source
parameters [e.g., Battaglia et al., 2008] and it is also
important for process identification and forecasting
volcanic behavior [e.g., Rymer and Williams-Jones,
2000]. This study combines time series data from
microgravity measurements (2002 to 2009) and
deformation data [Dzurisin et al., 2006, 2009] at
Three Sisters Volcanic Field (Oregon, USA) in
10.1029/2012GC004341
order to further constrain the nature of the inferred
intrusion at depth.
2. Geologic Setting
[5] The Three Sisters Volcanic Field is located in
central Oregon and is part of the Cascade Volcanic
arc, which stretches from northern California to
southwestern British Columbia (Figure 1). The
Juan de Fuca plate, at the Oregon coast, is subducting beneath North America obliquely at a rate
of 3 cm yr 1 [Riddihough, 1980; Bates et al.,
1981]. The central Oregon section of the volcanic
arc has produced more Cenozoic vents and lava
than any other part of the arc, while historically
producing very few seismic events [Guffanti and
Weaver, 1988; Priest, 1990; Sherrod and Smith,
1990; Nichols et al., 2011]. The region’s aseismic
behavior is likely due to a change in tectonic
stresses from compression to extension, as well as
increased heat flow. Blackwell et al. [1982, 1990]
show that the regional heat flow in the central
Oregon arc averages over 100 mW m 2, in comparison to 60 mW m 2 in the Washington section
of the arc.
[6] The region’s extensional stresses are a byproduct of rotation, as southern Washington and
Oregon rotate away from the rest of North America
[e.g., Magill et al., 1981]. A possible cause for the
apparent rotation is the collapse of the Basin and
Range [McCaffrey et al., 2000]. It is possible that
the rotation driven extension is responsible for the
high regional heat flow, gravity anomaly and
increase in eruptive products in the central Oregon
Cascades [Blackwell et al., 1982, 1990].
[7] Based on tephrachronology, dating and field
relationships, the Three Sisters Volcanic Field has
likely been a center of volcanic activity for longer
than 700,000 years [Scott et al., 2001]. There are five
large Quaternary age cones that dominate the area
including: North Sister (400–55 ka [Schmidt and
Grunder, 2009]), Middle Sister (37–14 ka [Calvert
et al., 2005]), South Sister (178–2 ka [Hildreth,
2007]), Broken Top (150–300 ka [Hildreth, 2007]),
and Mount Bachelor (18–8 ka [Scott et al., 1989;
Scott and Gardner, 1990, 1992]) (Figure 2). The
youngest, South Sister, erupted rhyolite tephra and
lava flows approximately 2000 years ago [Taylor,
1978; Wozniak, 1982; Clark, 1983; Scott, 1987;
Fierstein et al., 2011]. This long-lived volcanic
center has also had considerable eruptive activity
away from the main cones with 10s of vents erupting
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Figure 1. Subduction of the Juan de Fuca, Gorda and Explorer plates off the west coast of North America. Red triangles represent major volcanic centers in the Cascade Volcanic arc. Inset: North America with the highlighted area in
red representing the Cascade Volcanic arc.
Figure 2. Topographic map of the Three Sisters Volcanic Field. Grey circle represents the approximate area affected
by the current deformation event. Five large Quaternary cones are indicated with red triangles. The gravity network
stations are displayed as black squares and numbered: (1) BASE, (2) BUGS, (3) BRUCE, (4) CUT and (5) CENTER.
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Figure 3. Modeled decaying deformation curves for each gravity station [Dzurisin et al., 2009]. Shaded areas indicate individual gravity surveys.
over the last 4000 years [Scott et al., 2001]. The most
recent eruptive event away from the main cones
took place approximately 1500 years ago at Belknap
Crater, 20 km north of South Sister, with an eruption of basaltic and andesitic lavas [Fierstein et al.,
2011].
[8] Eruptive products, from basaltic to rhyolitic, and
different vent locations have led to a variety of
eruption styles at the Three Sisters Volcanic Field in
the past [Scott et al., 2001]. Large explosive eruptions are rare but have occurred at least 4 times in the
last 700,000 years [Scott et al., 2001], however, at
this time, there is no evidence of a magma chamber
of sufficient size to drive a large Plinian eruption.
The vent locations in the area and their link to tectonic stresses and general eruptive behavior have
been discussed by Bacon [1985], who suggests that
the last silicic eruptive episode was fed from a small
deep reservoir on the south side of South Sister. The
current unrest and deformation takes place west of
South Sister and appears to have no relation to the
most recent volcanic eruption.
Mogi point source model [Mogi, 1958; Wicks et al.,
2002]. Since the discovery of uplift in the Three
Sisters region, further deformation [Dzurisin et al.,
2006, 2009; Riddick and Schmidt, 2011] and water
geochemistry surveys [Evans et al., 2004] have
been completed in order to better characterize the
deep seated processes responsible for this activity.
[10] Spring geochemistry of the Three Sisters area
was first investigated by Ingebritsen et al. [1994]
who showed that there was a mantle-derived component of CO2 prior to the start of the current deformation event in the Separation creek drainage system
(Figure 2). The study also showed an anomalous
chloride load of 10 g s 1, suggesting that hydrothermal fluids were being incorporated into the
springs that drain into Separation creek. More recent
data from 2001 and 2002 [Evans et al., 2004], shows
that there was no change in the chloride load or in the
temperature of the water flowing in Separation creek,
suggesting that previous intrusive heat sources were
controlling the hydrothermal system near the center
of uplift.
[11] Dzurisin et al. [2006, 2009] refined the original
3. Previous Work
[9] Wicks et al. [2002] discovered that an area west
of South Sister was deforming and that it likely
started as early as 1996. The results of this study
show that from 1998 to October 2000, the deformation was steady with 3 to 5 cm yr 1 of uplift.
Modeling of the early results indicated a source
depth for the inferred intrusion at 6.5 km, based on a
deformation models of Wicks et al. [2002] with a
longer time series and more data from continuous
and campaign GPS, as well as leveling surveys.
Using 95% confidence levels, these deformation
models describe the source as a prolate spheroid with
a depth of 5 km. Dzurisin et al. [2009] suggested
that the uplift rate was decaying (Figure 3) and used
it to calculate bounds on the volume change for
the whole event by extrapolation; suggesting the
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total volume may reach 45 to 52 106 m3. They
put forward three conceptual models that could
explain the current deformation event: (1) hydraulic
or instantaneous response of the crust to continued
intrusion at depth; (2) pressurization of the hydrothermal system in the area of Three Sisters and
(3) continued viscoelastic response of the crust due
to an intrusion emplaced at depth. A more recent
study using InSAR data [Riddick and Schmidt,
2011] argues that a sill model is more appropriate
resulting in a deeper (7 km) and a larger source
(5–7 107 m3). This study better defines the initiation and rates of deformation up to 2001 and
suggests the presence of an inflection in the deformation rate in 2004; however, continuous GPS
data since 2004 is best fit by an exponential curve
(M. Lisowski, personal communication, 2012). Our
study uses the deformation results of Dzurisin et al.
[2006, 2009] (Figure 3).
4. Methodology and Results
4.1. Microgravity
[12] Mass flux and redistribution at depth can be
constrained using a combination of microgravity and
high resolution deformation surveys. The microgravity surveys implemented at Three Sisters measure small changes in the gravitational field over time
and space across a network of stations. The application and theory of this technique is discussed thoroughly in the literature [e.g., Eggers, 1987; Rymer
and Brown, 1986; Rymer, 1996; Battaglia et al.,
2008] and hence, will only be summarized here.
[13] Due to the remoteness and rugged nature of the
area, monitoring of only a very limited gravity network at Three Sisters was feasible. The network
consists of 5 stations making a single transect partly
spanning the deforming area, which is approximately 10 20 km (Figure 2). Measuring only a
single profile reduces our ability to describe the
source of the event, however, it does cover from the
edge to the center of the deformation zone. Repeat
measurements in part compensate for the poor spatial coverage. The station locations were chosen
such that there was less than 1 mGal difference from
the reference station outside the deforming area
with respect to the rest of the network to reduce
possible tares from having to re-level the beam
within LaCoste & Romberg gravimiters. This was
accomplished by utilizing both a topographic map
to identify areas of equal elevation along the trail
and field testing of these sites with a LaCoste &
Romberg spring gravity meter. Each station consists
10.1029/2012GC004341
of three metal rods drilled into bedrock and made
flush with the ground. The rods are arranged such
that the 3 levelling screws on LaCoste & Romberg
G- and D- meters rest on them to eliminate the need
for a base plate while ensuring precise positioning.
The stations from the edge of the deforming area
to the center are: BASE, BUGS, BRUCE, CUT,
and CENTER (Figure 2). CENTER is approximately located at the center of the deforming area,
near Separation creek; BASE is on the edge of the
deformation zone and is used as the reference station
since it is not expected to vary appreciably over the
period of study.
[14] To maximize accuracy, gravity measurements
were collected in station loops where each loop
repeats every station at least twice, except CENTER,
in order to pinpoint and correct for tares. Tares
represent high frequency noise in gravity data due
to changes in a gravimeter’s spring length, which
are typically caused by the instrument receiving a
physical shock. Tares in gravity data are either non
recoverable, represented by a simple offset between
two measurements, or recoverable where the spring
recovers over time to its original length. To further
allow for tare correction and the removal of noisy
data, surveys (station loops) were completed three
times over a period of 3 to 6 days; the exception is
2008 and 2009 where only one loop was completed
over a single day (Table 1). After corrections on
each survey were performed, surveys which were
completed over a single week were grouped and
averaged to represent one data point. Two gravity
meters were also used on each survey to eliminate
any bias due to anomalous results from instrumental malfunction or noise. In some survey sets, station loops were shortened such that stations could
be repeated more frequently to increase accuracy
(Table 1).
[15] The raw gravity data was first corrected for
the effects of Earth tides. The calculation of Earth
tides, based on the recorded time and position of
a gravity measurement, has accuracy better than
2 mGal (Quick Tide Pro [Micro-g LaCoste]).
Changes in a station’s vertical position were corrected by multiplying the height change with the
theoretical free air gradient ( 306.8 mGal m 1)
and subtracted. The vertical position of each station was obtained from deformation models and
nearby levelling lines [Dzurisin et al., 2009]; the
models have been shown to fit the deformation data
within 95% confidence levels (Figure 3). Instrumental factors such as drift can be ignored as the
meters used in this study were stable and the
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Table 1. Microgravity Data From 2002 to 2009 Normalized to the First Survey in August 2002
Normalized Gravity (mGal)
Survey Group
BASE
BUGS
BRUCE
CUT
CENTER
Instruments
2–8 Aug., 2002
2–4 Sept., 2002
17–19 Sept., 2002
15–18 Jul., 2004
30 Jul.–1 Aug., 2004
23–26 Aug., 2004
24 Sept.–1 Oct., 2004
28 Jun.–1 Jul., 2005
22–24 Aug., 2005
26–28 Sept., 2005
12 Oct., 2008
1 Sept., 2009
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
TAREa
0.0
TAREa
0.0
0.0
17
35
24
25
67
22
12
TAREa
74
TAREa
17
0.0
2
8
TAREa
TAREa
TAREa
41
5
TAREa
TAREa
TAREa
48
0.0
25
19
TAREa
13
6
4
11
TAREa
TAREa
TAREa
148
0.0
4
15
TAREa
TAREa
5
16
12
TAREa
32
TAREa
52
G-209, G-248
G-209, G-248
G-209, G-248
G-209, D-17
G-209, D-17
G-209, D-17
G-209, G-127
D-52, G-127
G-209, D-52
G-209, D-52
G-209, G-127
G-209, G-127
a
TARE: data were not used due to excessive high frequency noise.
instrumental drifts were negligible over the course
of a single survey.
[16] After corrections, the data were normalized
to the base station; BASE’s closure was averaged
and then subtracted from each other measurement
in the survey. In surveys that did not close with
BASE, the data were normalized to BASE, using a
normalized value of BRUCE. This was obtained
from other survey days in the same grouping where
both BRUCE and BASE were collected. With each
station normalized and repeated in a survey, it is
easy to identify when a data tare occurred. However,
correcting and removing the effects of tares is usually non trivial and increases the uncertainty of the
data. It is thus preferable to not use tare-corrupted
data, especially when attempting to obtain precise
values. The redundancy in the data, by having three
surveys with two meters for each data point in time,
allows for strict quality control and as such, the data
were not corrected for tares. If an unrecoverable tare
occurred, either the data from the whole survey with
that meter was discarded or only the data before the
tare were used. If there was a recoverable tare such
that the meter did not display an offset following
it, measurements were thrown out until a reasonable
closure between stations was obtained (less than
25 mGal). The upper limit for closures from the first
to last measurement of the survey is 60 mGal if no
specific tare can be identified. Averaging the results
from both meters and the whole survey group significantly reduces the effect of any larger closures.
[17] With each station within 1 mGal of the next,
calibration between meters is less of a concern, after
normalizing each survey to the reference station.
The two meters that were used most frequently were
G-209 and G-127. G-209, G-248 and D-52 were
analogue (with optics and calibration dial) and to
be consistent, each analogue meter had the same
operator making the measurements at each station.
Meters G-127 and D-17 utilize a digital Aliod
feedback system to record and collect field measurements. This system allows a 100 mGal dynamic
range and removes the necessity for a surveyor
to manually null the meter. It also streams the data
to a serial device at 2 Hz; each reading in the survey
was averaged over one minute, with at least
5 readings per station to further reduce noise. The
change in the calibration factor for each G meter is
on the order of 0.01 for each 100 mGal of gravity
change; therefore, a change of less than 1 mGal
is of the order of 1 10 4. The calibration factor
between G-127 and G-209 has been determined
to be 0.09899 or approximately 10 mGal for every
1 mGal of change. This allows for the data of
multiple meters to be used and merged without
having to carefully inter-calibrate each meter.
[18] It is possible to obtain measurement accuracies
of 10 mGal in volcanic areas if strict survey procedures are used [e.g., Rymer and Brown, 1986;
Rymer, 1994]. However, as mentioned above, the
Three Sisters station network is not in an ideal
environment for measuring gravity. The major difficulty with processing this data set is the presence
of tares, particularly in 2005 (Table 1). In 2004
and 2005, D-meters were used and frequently had
large tares. In addition to sensitive meters, the trail
used to access the gravity network is approximately
20 km in length and takes 10 to 12 h to complete
on foot over rugged ground. The constant jostling
of walking, even with the spring clamped, can
create tares [e.g., Crider et al., 2008]. The data
collected in 2002 is some of the cleanest in this
study, with very few obvious tares corrupting
the surveys. High frequency noise throughout the
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Figure 4. Residual gravity data for BUGS (red), BRUCE (purple), CUT (green), and CENTER (brown) from
2002 to 2009. Error bars are the standard deviation of repeat measurements at each point and the highlighted region
is +/ 25 mGal from 0. Dashed lines refer to expected changes due to a viscoelastic response causing a decrease in
the gravitational field.
survey is, however, still prevalent, as the closures
reached up to 56 mGal. By using repeats and
redundant data, much of the noise is averaged out
through multiple surveys; as a result, no data were
excluded in 2002. The data collected in 2004, 2005
and 2008, however, had many large tares and in
some cases, the data was completely unusable. In
2004 and 2005, 50% and 40% of the data, respectively, were unusable due to tares. There were
24 survey loops (12 surveys) completed in each of
these years, so although large amounts of the data
were unusable, there is no loss of information about
the gravitational field. The data collected in 2008
had to be thrown out completely, while a tare-free
survey was collected in 2009. The standard deviation
is taken as the estimated error for each data point
averaged from a survey group. Where there are
insufficient measurements to effectively calculate the
standard deviation, an error of 25 mGal is assumed.
The normalized residual gravity through time does not
change appreciably as the majority of the measurements are within one standard deviation (Figure 4).
The station BUGS has one point which is lower than
the error bounds in 2005, while all other stations have
a lower outlier in 2009.
4.2. Viscoelastic and Gravity Modeling
[19] In order to test the validity of the hypotheses
proposed by Dzurisin et al. [2006, 2009], modeling
of both gravity and viscoelastic response of the crust
was performed. A gravity forward modeling program
[GRAV3D, 2007] (Figure 5) was used in conjunction
with volumetric results from 2002 to 2009 [Dzurisin
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Figure 5. A forward gravity model using 2900 kg m 3 as the melt density, overlain by 100 m topography contours.
Numbered black squares represent gravity stations: (1) BASE; (2) BUGS; (3) BRUCE; (4) CUT; (5) CENTER.
et al., 2006, 2009; Riddick and Schmidt, 2011] to
predict the gravitational field caused by continual
intrusion at depth. Dzurisin et al. [2006] calculated
the yearly volumetric change using their deformation model to be 3.5 to 6.5 106 m3 yr 1. Extrapolating the yearly volumetric increase from 2002
to 2009 gives a total expected volume change of
24.5 to 45.5 106 m3. In a later study, deformation
models were updated with more data [Dzurisin et al.,
2009]; however, the modeled source volumes are
within the original volumetric bounds put forth
by Dzurisin et al. [2006]. To eliminate any bias or
uncertainty in the possible model geometries, the
intrusion was modeled from 5 to 6 km depth with
a volume of 2.4 to 6.5 107 m3. The intrusion is
assumed to have approximately the same density
as the crust, which would be the case if the intrusion
stalled due to buoyancy forces. The addition of
material to the system is modeled as an increase
in mass as a Mogi [1958] point source. Direct
comparisons to the gravity data must be made only
on data that has been corrected for vertical change.
[20] The gravity point source models show a 20 to
33 mGal increase in the gravitational field near
the station CENTER from 2002 to 2009. They predict that the maximum magnitude of the gravity
increase will fall away from CENTER, with
CUT having a maximum increase of 25 mGal and
BRUCE of 16 mGal (Figure 5). A sensitivity analysis of the intrusion shape and density was performed to test what effect initial assumptions have
using forward gravity modeling software GRAV3D
[GRAV3D, 2007]. Modeling the intrusive source as
rectangular or spherical shapes made no appreciable
difference in the resulting gravitational field.
[21] Deformation modeling with a geomechanics
software package, FLAC 6.0 (Fast Lagrangian Analysis of Continua [Itasca Consulting Group, 2007]),
was used to test the possibility that viscoelastic
response of the crust is the controlling factor in
the observed deformation. FLAC 6.0 allows for
the analysis of stress fields through elastic, viscoelastic and viscoplastic modeling in two dimensions.
It has been used previously in volcanic studies,
however, mostly in the application to slope stability
and flank collapse [e.g., Apuani and Corazzato, 2005,
2009; Casagli et al., 2009]. In this study, only elastic
and Maxwell viscoelastic materials were considered
when attempting to model the deformation event at
Three Sisters Volcanic Field. There are very few
constraints available to obtain realistic properties for
crustal material thus assumptions about viscosity,
density, bulk and shear modulus have to be made.
Due to the wide range of eruption types, geologic
processes and heterogeneity, it is impossible to
accurately represent the crust in the Three Sisters
area. Therefore, a simple model with the elastic
properties of basalt was used (Table 2), as basalt to
basaltic andesite volcanics are the most common
eruptive products near the center of uplift [e.g.,
Wozniak 1982; Taylor, 1987] and old intrusions at
depth would presumably also be basaltic in composition. High heat flow in the Three Sisters region
[e.g., Blackwell et al., 1982, 1990; Bonafede et al.,
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Table 2. Average Elastic Properties for Basalt Used
in Viscoelastic Modeling of the Crust
Property
Value
Density
Bulk modulus
Shear modulus
2700 kg m 3
3.32 1010 Pa
1.32 1010 Pa
1986] suggests that viscous deformation cannot be
ignored; however, there is no rigorous method to
accurately constrain the crustal viscosity at depth.
The only values of crustal viscosities that have been
inferred are derived from post-seismic deformation
and isostatic rebound for the upper mantle and the
lower crust [e.g., Wdowinski and Axen, 1992; Ueda
et al., 2003]. Values from these studies range from
3 1017 to 1 1018 Pa s for the upper mantle and
1 1018 to 1 1023 Pa s for the lower crust.
10.1029/2012GC004341
stop the edges from deforming outward, while
the top is allowed to deform freely. The models have
a dynamic viscosity range of 1018 to 1020 Pa s
and show two end-members of the deformation
response (Figure 6). For each viscosity, the model
run time was 30 years using a time step of 1 day.
Models with viscosities 1020 Pa s or higher are
dominated by a nearly instantaneous elastic deformation with a small viscoelastic component, while
models with viscosities on the order of 1018 Pa s
are dominated by a linear viscoelastic response.
Results show that a viscosity of 1018 Pa s can produce the observed uplift after the elastic response
has ceased. However, for a viscosity of 1020 Pa s,
there is essentially no viscoelastic response, thus it
cannot produce the continual deformation at rates
observed, unless there is a continuous injection of
material at depth.
[22] The two dimensional viscoelastic model used
here is 6 km deep and 6 km wide consisting of a flat
elastic top layer 4.5 km thick and a bottom viscoelastic layer 1 km thick. The intrusion is represented
by an applied constant upward vertical force at 5 km
depth across a small 100 m long surface. The side
and bottom boundaries of the model are fixed to
5. Discussion
5.1. Crustal and Magmatic Properties
[23] The deformation episode at the Three Sisters
began in 1997 [Dzurisin et al. 2009] and continues
Figure 6. Modeled viscoelastic responses of the crust for different viscosities with their corresponding applied force.
In each case, the elastic properties were that of basalt (see Table 1).
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Figure 7. Schematic model to explain the activity to the west of South Sister. (a) Viscoelastic model with a continual
response of the crust to past overpressure. (b) Hydraulic model with a continual response of the crust to pressure from
intrusive injection of material. (c) Hydrothermal pressurization causing continual uplift.
at a declining rate to the present. Previous work
has outlined three possible end-member models
that can explain the deformation event (Figure 7).
(1) Continuous injection of material at depth where
the magma flow rate from the lower crust is proportional to the pressure causing uplift. (2) Instantaneous pressurization of the crust and the time
dependent response of a Maxwell fluid causing
continued uplift at the surface. (3) Pressurization
of hydrothermal fluids due to the injection of
magmatic volatiles from a previous crustal magma
body. Each of these possibilities is discussed below
with reference to the expected gravitational field
and models.
the auxiliary material).1 If water tables are a controlling factor, a positive gravity anomaly would be
expected in 2004. The corrected data shows no
change throughout the data set greater then estimated error for the surveys (Figure 4). There is,
however, an exception where small gravity decreases were measured at stations CUT and CENTER in
2009. The precipitation in 2009 is comparable to
that in 2002 and thus a gravity decrease from 2002
to 2009 based on water table changes is not expected. Since the gravity signal has no measurable
change outside the estimated error in both 2004 and
2005, we assume that changes in groundwater tables
have no obvious influence on the gravity data set.
[24] In order to interpret the microgravity data span-
[25] Within the microgravity data set, there are 4
ning the deforming area at Three Sisters, the effect of
water table fluctuations must first be addressed. The
surveys performed in 2004 and 2005 occurred from
early summer into fall, in an effort to characterize
the seasonal effects in groundwater levels. However,
the large number of tares and loss of data limit the
effectiveness of this approach. Instead, it is better to
analyze the data as a whole in conjunction with
monthly precipitation as it is more robust and less
sensitive to a single anomalous survey. The precipitation data shows that 2004 had more rain than any
other year during the survey months (Figure S1 in
points that fall outside the estimated error, 3 of
them in 2009. Each shows a smaller gravitational
field than what was originally measured in 2002.
The 2009 decrease at CUT could be wholly or
partially attributed to unobserved mass wasting as
the station is located near a small 5 to 10 m high
cliff; a failure of only 125 m3 could explain the
observed gravity decrease (150 mGal). This cannot, however, explain a smaller decrease at both
1
Auxiliary materials are available in the HTML. doi:10.1029/
2012GC004341.
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BRUCE and CENTER stations in 2009. If the
gravitational field is solely created through viscoelastic deformation, then the crustal density is
effectively decreasing due to expansion. The total
deformation from 2002 to 2009 would result in a
16 to 24 mGal drop in the gravitational field
at CENTER, 14 to 19 mGal at CUT, 13
to 10 mGal BRUCE, 3 mGal at BUGS and
1 mGal at BASE. If this correction due to viscoelastic expansion is applied to the residual gravity
data for CENTER and BRUCE, the values would
fall within error around zero (Figure 3 and Text S1).
Regardless, it is clear that there is no observed
increase in the gravitational field which would be
expected from a positive mass flux.
[26] The data collected by Evans et al. [2004] show
that the geochemical anomalies in the Separation
creek drainage are most likely caused by previous
intrusions and not directly connected with the current event. If hydrothermal pressurization is the
main cause of deformation then it would also be
expected to increase the amount of hydrothermal
fluids at depth. Residual gravity would then increase
as hydrothermal fluids add mass to the system.
However, depending on the volume and depth, this
mass increase may be too small to detect above the
noise in the gravity data. The steady state nature of
the geochemical results suggests that any hydrothermal pressurization would have to be deep
enough not to interact with groundwater. The depth
of the deformation source at 4.9 km inferred by
Dzurisin et al. [2009] is sufficiently deep that there
is essentially 0% porosity, suggesting that at this
depth, hydrothermal fluids cannot be the cause of
uplift. While the models and data presented do not
sufficiently constrain the deformation and mass
flux to completely rule out hydrothermal pressurization as a possible cause, they do suggest that this
is the least likely conceptual model to explain the
current deformation event at the Three Sisters Volcanic field.
[27] Gravity forward modeling shows that the
expected gravitational increase, if continual injection of material is the sole process, is between 20 to
33 mGal. Field measurements, however, do not
show any increase and may suggest a decreasing
gravitational field near CENTER and CUT. The
lack of gravitational increase suggests that a continual flow of material at depth into an intrusion is
not responsible for the deformation. The amplitude
of the modeled gravitational field, however, straddles the error levels of the microgravity data set.
Hence, it is not possible to completely rule out the
possibility that the crust is behaving hydraulically as
10.1029/2012GC004341
material is injected continuously at depth. It does
suggest, though, that viscoelastic response of the
crust is at least partially responsible for the continual
uplift.
[28] The increased heat flow for the central Oregon
section of the Cascade arc could play a dominant
role in determining how the crust is responding to
the inferred deep-seated source. Blackwell et al.
[1990] made measurements of heat flow throughout much of Oregon and although the measurements
do not cover the Three Sisters region, they do provide an estimate of the heat flow and geothermal
gradient. An average of 65 C km 1 was obtained
along arc near volcanic centers but the values could
be much higher near the main cones, to as much as
100 C km 1 [Blackwell et al., 1982, 1990]. This
would suggest that the crust around the inferred
intrusion at 5 km depth could be between 325 and
500 C. The strength of quartz greatly decreases at
temperatures greater than 350 C [e.g., Buck, 1991]
and incoherent material can reduce the effective
viscosity [Bonafede et al., 1986]. While these
properties are known, there have been no rigorous
studies to obtain a crustal viscosity in local areas of
high heat flow. Therefore, any viscosity used is
conjecture based on studies that infer lower crustal
viscosities from post seismic relaxation (e.g., 4 1018 Pa s, Japan [Ueda et al., 2003]). The temperature may be similar in volcanic areas to that in the
lower crust; however, the confining pressure is significantly less.
[29] Furthermore, there is also a conspicuously low
level of seismic activity in the central Oregon part
of the Cascade Volcanic Range [Weaver and
Michaelson, 1985]. This includes the current
deformation event at Three Sisters as there have
been very few seismic events associated with it,
apart from a seismic swarm in 2004 that consisted
of over 300 seismic events [Dzurisin et al., 2006].
High heat flow, low levels of seismic activity, and
the presence of incoherent material suggest it is
not unreasonable that the crust beneath the Three
Sisters could have viscosities lower than 1020 Pa s.
[30] If deformation is solely caused by viscoleastic
response of the crust, there would be no additional
mass added to the system. Therefore, if the observed
deformation from 2002 to 2009 is primarily caused
by a viscoelastic response due to an intrusion, the
results of the microgravity data would show no net
increase after being corrected for changes in elevation. Furthermore, viscoelastic models provide
limited bounds on the expected viscosity and suggest the possibility that the deformation event is a
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hybrid of viscoelastic response and continual
injection. Modeling shows that it is possible to
obtain the observed rate of deformation with a
nearly instantaneous intrusion of magma if the
crustal viscosity beneath the Three Sisters is on the
order of 1018 to 5 1019 Pa s. Viscosities that are an
order of magnitude higher have a strong elastic
component to the deformation and hence would
require some magmatic injection to continue while
also deforming viscoelasticly. It should be noted
that the modeled response to the intrusion does not
decay (Figure 6). This is due to the intrusion being
modeled as a constant force and not a realistic
intrusion where the force decays and generates an
exponentially decaying deformation field. The
gravity data does not rule out this possibility as
material injected in this way would be below the
detection limits of the survey. If viscosities reach
1021 Pa s, the modeled deformation is nearly all
elastic and would require continual injection of
material. The gravity data does not show an increase
hence it does not support continual injection as the
sole cause of the continuing deformation event as
previously discussed. The viscosity, as determined
from these simple models for the crust beneath
the Three Sisters, is most likely between 1018 and
5 1019 Pa s with the deformation from 2002 to
2009 being either dominated by a viscoelastic
response or a combination of elastic and viscoelastic. The viscosity range is much lower than expected
for upper crustal rocks, however, it falls within the
range of that expected for the lower crust [e.g.,
Wdowinski and Axen, 1992].
5.2. Magma Emplacement Through
Density Sieving
[31] If the viscosity beneath the Three Sisters vol-
canic field is low (10 to 5 10 Pa s at 5 km), it
must have a profound impact on magma rise and
emplacement. It has been suggested that the locations of magma chambers at other volcanic centers
are controlled by the ductile-brittle transition zone
[e.g., Burov et al., 2003]. High heat flow and low
crustal viscosity allows buoyancy forces to be the
defining factor if or where magma will stall at
depth. If buoyancy forces dictate where magma will
stall and intrusion rates are relatively low, one
would expect a series of magma bodies with depths
dependant only on the density of the rising fluid
like a density sieve.
18
19
[32] A geochemical study on South Sister shows
that there are two compositional gaps between 56–
10.1029/2012GC004341
62% SiO2 and 66–73% SiO2 [Brophy and Dreher,
2000]. The study argues that the compositional
gaps are created through magmas stalling at depth,
fractionating to higher silica melts and then breaking through the top of the crystallizing intrusion
to rise, stall and fractionate further. A similar
model has been proposed for Medicine Lake volcano in California [Brophy et al., 1996] based on the
compositional gaps found there. Fractional crystallization fits the whole rock geochemistry; however,
with crustal viscosity so low, magma should continue to rise as it fractionates and becomes less
dense, instead of stalling. More recently, U-series
dating together with trace element data on plagioclase and zircon crystals for two different eruptions
on South Sister suggest a different explanation.
The two eruptions occurred 2000 and 2300
years ago and while they both show similar crystallization ages, they have distinct trace element
concentrations [Stelten and Cooper, 2012]. The
authors suggest that melt for these two eruptions
must have been derived from different source
regions and evolved separately. Furthermore, other
geochemical data show compositional gaps between
59 to 66% SiO2 in melt inclusions from many volcanic arcs, even where volcanoes erupt a continuous
spectrum of compositions [e.g., Naumov et al.,
1997; Reubi and Blundy, 2009]. Due to the lack of
andesitic melt inclusions and laboratory experiments, it is suggested that arc compositional gaps
form due to differences in melt source. The two
general melt source regions that have been used to
explain the bimodal distribution are a mantel source
that produces mafic magmas and melting in the deep
to mid-crustal levels that produces silicic magmas
[e.g., Reubi and Blundy, 2009; Kent et al., 2010].
[33] Intrusion rates at South Sister have been esti-
mated at 0.0003 km3 yr 1 [Evans et al., 2004] which
is an order of magnitude lower than other volcanoes
like Mount St. Helens and Mount Fuji [Crisp, 1984;
Sherrod and Smith, 1990]. Low intrusion rates will
reduce the possibility that magmas will have the
opportunity to vigorously mix and produce a continuous eruptive chemical spectrum from basalt to
rhyolite, leaving compositional gaps recorded in the
whole rock chemistry. While there has not been a
detailed melt inclusion study on South Sister, the
high heat flow, low viscosity, low intrusion rate and
geochemical evidence at other volcanoes, suggest
that different melt sources are responsible for the
composition gaps. Furthermore, low crustal viscosity acts as a density sieve dictating where magma
will stall. This model should also be applicable to
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other arc setting volcanoes with high heat flow and
low intrusion rates.
6. Conclusion
[34] Three general conceptual models have been
suggested by past studies [Dzurisin et al., 2006,
2009] to explain the cause of the deformation event at
the Three Sisters Volcanic complex: (1) viscoelastic
response of the crust due to instantaneous pressurization from an intrusion; (2) continual intrusion of
material at depth causing constant deformation;
(3) overpressure caused by many shallow hydrothermal sources. Spring geochemistry studies indicated that there has been no measurable change in the
hydrothermal system following the start of the
deformation event [Ingebritsen et al., 1994; Evans
et al., 2004] suggesting that model 3 is the least
likely. Models 2 and 3 require a positive mass flux to
drive uplift from 2002 to 2009, whereas viscoelastic
deformation (model 1) does not. Furthermore, the
uplift event has been generally aseismic suggesting
that viscoelastic deformation probably plays a major
role. Microgravity surveys were completed to constrain the deformation process by determining
whether any mass was added beneath the deforming
area. Gravity results show no significant change,
within error, in the mass flux across the deforming
area, suggesting that the crust is deforming viscoelastically. While it is impossible to quantify the
amount of deformation that could be attributed to
viscoelastic response due to noise within the gravity
surveys, it is possible to provide some constraints on
the crustal viscosity in the Three Sisters region.
Viscosities greater than 5 1019 Pa s have a very
weak viscoelastic component requiring the constant
addition of material to drive deformation, however,
if the crustal viscosity is 1018 Pa s, then it is possible
to continue the deformation event from a single
instantaneous pressurization. The most likely cause
of this deformation event is the combination of
hydraulic and viscoelastic responses due to a magmatic intrusion with average crustal viscosities
beneath the Three Sisters between 5 1019 Pa s and
1018 Pa s. This agrees with previous studies that the
most likely cause of the deformation event is the
intrusion of magma at depth. The suggested low
crustal viscosity also argues for density and buoyancy
forces as the controlling factor for magma emplacement beneath South Sister. To further ground truth
both the geothermal gradient and crustal viscosity, a
set of drill holes could be made or an active seismic
survey could be performed. This would provide better
constraints for building a comprehensive model of
10.1029/2012GC004341
this continual process, however, the area is a protected wilderness and these approaches are invasive.
Unless there is reactivation with a larger intrusion to
reverse the decaying deformation trend, this event is
extremely unlikely to lead to an eruption.
Acknowledgments
[35] Financial support for this study came from a NSF grant
EAR-0208490 to D. Johnson and a NSERC Discovery grant
to G. Williams-Jones. Special thanks to Dan Dzurisin, Mike
Poland, Nico Fournier, Hazel Rymer, Doug Stead, and Patricia
MacQueen for their discussions and support. Thanks also to the
G-Cubed editor, James Tyburczy, the associate editor, Michael
Lisowski, Giovanna Berrino and two anonymous reviewers for
their thoughtful and constructive suggestions.
References
Apuani, T., and C. Corazzato (2009), Numerical model of the
Stromboli volcano (Italy) including the effect of magma pressure in the dyke system, Rock Mech. Rock Eng., 42(1), 53–72,
doi:10.1007/s00603-008-0163-1.
Apuani, T., C. Corazzato, A. Cancelli, and A. Tibaldi (2005),
Stability of a collapsing volcano (Stromboli, Italy): Limit
equilibrium analysis and numerical modeling, J. Volcanol.
Geotherm. Res., 144(1–4), 191–210, doi:10.1016/j.jvolgeores.
2004.11.028.
Bacon, C. R. (1985), Implications of silicic vent patterns for
the presence of large crustal magma chambers, J. Geophys.
Res., 90, 11,243–11,252, doi:10.1029/JB090iB13p11243.
Bates, R. G., M. E. Beck, and R. F. Burmester (1981), Tectonic
rotations in the Cascade Range of southern Washington,
Geology, 9(4), 184–189, doi:10.1130/0091-7613(1981)
9<184:TRITCR>2.0.CO;2.
Battaglia, M., P. Segall, and C. Roberts (2003), The mechanics
of unrest at Long Valley caldera, California. 2. Constraining
the nature of the source using geodetic and micro-gravity
data, J. Volcanol. Geotherm. Res., 127(3–4), 219–245,
doi:10.1016/S0377-0273(03)00171-9.
Battaglia, M., J. Gottsmann, D. Carbone, and J. Fernàndez (2008),
4D volcano gravimetry, Geophysics, 73(6), WA3–WA18,
doi:10.1190/1.2977792.
Berrino, G. (1994), Gravity changes induced by heightmass variations at the Campi Flegrei caldera, J. Volcanol.
Geotherm. Res., 61(3–4), 293–309, doi:10.1016/0377-0273
(94)90010-8.
Berrino, G., G. Corrado, G. Luongo, and B. Toro (1984),
Ground deformation and gravity changes accompanying the
1982 Pozzuoli uplift, Bull. Volcanol., 47(2), 187–200,
doi:10.1007/BF01961548.
Berrino, G., H. Rymer, G. C. Brown, and G. Corrado (1992),
Gravity-height correlations for unrest at calderas, J. Volcanol.
Geotherm. Res., 53(1–4), 11–26, doi:10.1016/0377-0273(92)
90071-K.
Blackwell, D., R. Bowen, D. Hull, J. Riccio, and J. Steele
(1982), Heat flow, arc volcanism, and subduction in Northern
Oregon, J. Geophys. Res., 87(B10), 8735–8754, doi:10.1029/
JB087iB10p08735.
Blackwell, D., J. Steele, C. Frohme, G. Murphey, G. R. Priest,
and G. Black (1990), Heat flow in the Oregon Cascade Range
and its correlation with regional gravity, Curie point depths,
13 of 15
Geochemistry
Geophysics
Geosystems
3
G
ZUREK ET AL.: INFLATION AT THREE SISTERS USING GRAVITY
and geology, J. Geophys. Res., 95(B12), 19,475–19,493,
doi:10.1029/JB095iB12p19475.
Bonafede, M., and M. Mazzanti (1998), Modelling gravity
variations consistent with ground deformation in the Campi
Flegrei caldera (Italy), J. Volcanol. Geotherm. Res., 81(1–2),
137–157, doi:10.1016/S0377-0273(97)00071-1.
Bonafede, M., M. Dragoni, and F. Quareni (1986), Displacement and stress fields produced by a centre of dilation and
by a pressure source in a viscoelastic half-space: Application
to the study of ground deformation and seismic activity at
Campi Flegrei, Italy, Geophys. J. R. Astron. Soc., 87(2),
455–485, doi:10.1111/j.1365-246X.1986.tb06632.x.
Brophy, J. G., and S. T. Dreher (2000), The origin of composition
gaps at South Sister volcano, central Oregon: Implications for
fractional crystallization processes beneath active calc-alkaline
volcanoes, J. Volcanol. Geotherm. Res., 102(3–4), 287–307,
doi:10.1016/S0377-0273(00)00192-X.
Brophy, J. G., M. J. Dorais, J. Donnelly Nolan, and B. S.
Singer (1996), Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain, Medicine Lake
volcano, California: Implications for fractionation mechanisms and the formation of composition gaps, Contrib. Mineral. Petrol., 126, 121–136, doi:10.1007/s004100050239.
Buck, W. R. (1991), Modes of continental lithospheric extension, J. Geophys. Res., 96, 20,161–20,178, doi:10.1029/
91JB01485.
Burov, E., C. Jaupart, and L. Guillou-Frottier (2003), Ascent
and emplacement of buoyant magma bodies in brittle-ductile
upper crust, J. Geophys. Res., 108(B4), 2177, doi:10.1029/
2002JB001904.
Calvert, A. T., J. Fierstein, and W. Hildreth (2005), High precision argon dating at young arc volcanoes: Understanding
the past 40 kyr at Middle Sister, Or., Geochim. Cosmochim.
Acta, Suppl., 69(10), A269.
Casagli, N., A. Tibaldi, A. Merri, C. Del Ventisette, T. Apuani,
L. Guerri, J. Fortuny-Guasch, and D. Tarchi (2009), Deformation of Stromboli Volcano (Italy) during the 2007 eruption
revealed by radar interferometry, numerical modelling and
structural geological field data, J. Volcanol. Geotherm. Res.,
182(3–4), 182–200, doi:10.1016/j.jvolgeores.2009.01.002.
Clark, J. G. (1983), Geology and petrology of South Sister
volcano, High Cascade Range, Oregon, PhD thesis, Dep. of
Geol. Sci., Univ. of Oreg., Eugene.
Crider, J. G., K. Hill Johnsen, and G. Williams-Jones (2008),
Thirty-year gravity change at Mount Baker Volcano, Washington,
USA: Extracting the signal from under the ice, Geophys. Res.
Lett., 35, L20304, doi:10.1029/2008GL034921.
Crisp, J. A. (1984), Rates of magma emplacement and volcanic
output, J. Volcanol. Geotherm. Res., 20(3–4), 177–211,
doi:10.1016/0377-0273(84)90039-8.
Dvorak, J. J., and G. Berrino (1991), Recent ground movement
and seismic activity in Campi Flegrei, Southern Italy: Episodic growth of a resurgent dome, J. Geophys. Res., 96(B2),
2309–2323, doi:10.1029/90JB02225.
Dzurisin, D., J. C. Savage, and R. O. Fournier (1990), Recent
crustal subsidence at Yellowstone Caldera, Wyoming, Bull.
Volcanol., 52(4), 247–270, doi:10.1007/BF00304098.
Dzurisin, D., K. M. Yamashita, and J. W. Kleinman (1994),
Mechanisms of crustal uplift and subsidence at the Yellowstone caldera, Wyoming, Bull. Volcanol., 56(4), 261–270,
doi:10.1007/BF00302079.
Dzurisin, D., M. Lisowski, C. W. Wicks, M. P. Poland, and
E. T. Endo (2006), Geodetic observations and modeling
of magmatic inflation at the Three Sisters volcanic center,
central Oregon Cascade Range, USA, J. Volcanol.
10.1029/2012GC004341
Geotherm. Res., 150(1–3), 35–54, doi:10.1016/j.jvolgeores.
2005.07.011.
Dzurisin, D., M. Lisowski, and C. W. Wicks (2009), Continuing inflation at Three Sisters volcanic center, central Oregon
Cascade Range, USA, from GPS, leveling, and InSAR observations, Bull. Volcanol., 71(10), 1091–1110, doi:10.1007/
s00445-009-0296-4.
Eggers, A. A. (1987), Residual gravity changes and eruption
magnitudes, J. Volcanol. Geotherm. Res., 33(1–3), 201–216,
doi:10.1016/0377-0273(87)90062-X.
Evans, C. W., M. C. van Soest, R. H. Mariner, S. Hurwitz,
S. Ingebritsen, C. W. Wicks, and M. E. Schmidt (2004),
Magmatic intrusion west of Three Sisters, central Oregon,
USA; the perspective from spring geochemistry, Geology,
32(1), 69–72, doi:10.1130/G19974.1.
Fierstein, J., W. Hildreth, and A. T. Calvert (2011), Eruptive history of South Sister, Oregon Cascades, J. Volcanol. Geotherm.
Res., 207(3–4), 145–179, doi:10.1016/j.jvolgeores.2011.06.003.
Gottsmann, J., A. Folch, and H. Rymer (2006), Unrest at
Campi Flegrei: A contribution to the magmatic versus hydrothermal debate from inverse and finite modeling, J. Geophys.
Res., 111, B07203, doi:10.1029/2005JB003745.
GRAV3D (2007), A program library for forward modeling and
inversion of gravity data over 3D structures: Developed
under the consortium research project Joint/Cooperative
Inversion of Geophysical and Geological Data, UBCGeophysical Inversion Facility, Dep. of Earth and Ocean,
Univ. of B. C., Vancouver, Canada.
Guffanti, M., and C. S. Weaver (1988), Distribution of late Cenozoic volcanic vents in the Cascade Range: Volcanic arc segmentation and regional tectonic considerations, J. Geophys.
Res., 93, 6513–6529, doi:10.1029/JB093iB06p06513.
Hildreth, W. (2007), Quaternary magmatism in the Cascades:
Geologic perspectives, U.S. Geol. Surv. Prof. Pap., 1744,
136 pp.
Ingebritsen, S., H. R. Mariner, and D. R. Sherrod (1994),
Hydrothermal systems of the Cascade Range, north-central
Oregon, U.S. Geol. Surv. Prof. Pap., 1044-L, 85 pp.
Itasca Consulting Group (2007), FLAC modeling code version
6.00, software, Minneapolis, Minn. [Available at http://www.
itascacg.com/flac/index.php.]
Kent, A. J. R., C. Darr, A. M. Koleszar, M. J. Salisbury, and
K. M. Cooper (2010), Preferential eruption of andesitic magmas through recharge filtering, Nat. Geosci., 3, 631–636,
doi:10.1038/ngeo924.
Magill, J., A. Cox, and R. Duncan (1981), Tillamook Volcanic
Series: Further evidence for tectonic rotation of the Oregon
Coast Range, J. Geophys. Res., 86(B4), 2953–2970,
doi:10.1029/JB086iB04p02953.
McCaffrey, R., M. D. Long, C. Goldfinger, P. C. Zwick, J. L.
Nabelek, C. K. Johnson, and C. Smith (2000), Rotation and plate
locking at the southern Cascadia subduction zone, Geophys.
Res. Lett., 27(19), 3117–3120, doi:10.1029/2000GL011768.
Mogi, K. (1958), Relations between the eruptions of various
volcanoes and the deformation of the ground surfaces around
them, Bull. Earthquake Res. Inst. Univ. Tokyo, 36, 99–134.
Naumov, B. V., V. I. Kovalenko, A. D. Babansky, and M. L.
Tolstykh (1997), Genesis of andesites: Evidence from studies
of melt inclusions in minerals, Petrology, 5, 586–596.
Nichols, M. L., S. D. Malone, S. C. Moran, W. A. Thelen, and
J. E. Vidale (2011), Deep long-period earthquakes beneath
Washington and Oregon volcanoes, J. Volcanol. Geotherm.
Res., 200(3–4), 116–128, doi:10.1016/j.jvolgeores.2010.12.005.
Orsi, G., S. M. Petrazzuoli, and K. Wohletz (1999), Mechanical and thermo-fluid behaviour during unrest at the Campi
14 of 15
Geochemistry
Geophysics
Geosystems
3
G
ZUREK ET AL.: INFLATION AT THREE SISTERS USING GRAVITY
Flegrei caldera (Italy), J. Volcanol. Geotherm. Res., 91(2–4),
453–470, doi:10.1016/S0377-0273(99)00051-7.
Priest, G. R. (1990), Volcanic and tectonic evolution of the
Cascade Volcanic Arc, central Oregon, J. Geophys. Res.,
95(B12), 19,583–19,599, doi:10.1029/JB095iB12p19583.
Reubi, O., and J. Blundy (2009), A dearth of intermediate melts
at subduction zone volcanoes and the petrogenesis of arc andesites, Nature, 461, 1269–1273, doi:10.1038/nature08510.
Riddick, S. N., and D. A. Schmidt (2011), Time-dependent
changes in volcanic inflation rate near Three Sisters, Oregon,
revealed by InSAR, Geochem. Geophys. Geosyst., 12,
Q12005, doi:10.1029/2011GC003826.
Riddihough, R. P. (1980), Gorda plate motions from magnetic
anomaly analysis, Earth Planet. Sci. Lett., 51(1), 163–170,
doi:10.1016/0012-821X(80)90263-0.
Rymer, H. (1994), Microgravity change as a precursor to volcanic activity, J. Volcanol. Geotherm. Res., 61(3–4), 311–328,
doi:10.1016/0377-0273(94)90011-6.
Rymer, H. (1996), Microgravity monitoring, in Monitoring and
Mitigation of Volcano Hazards, edited by R. Scarpa and R. I.
Tilling, pp. 169–197, Springer, Berlin, doi:10.1007/978-3642-80087-0_5.
Rymer, H., and G. C. Brown (1986), Gravity fields and the
interpretation of volcanic structures: Geological discrimination and temporal evolution, J. Volcanol. Geotherm. Res.,
27(3–4), 229–254, doi:10.1016/0377-0273(86)90015-6.
Rymer, H., and G. Williams-Jones (2000), Volcanic eruption
prediction: Magma chamber physics from gravity and
deformation measurements, Geophys. Res. Lett., 27(16),
2389–2392, doi:10.1029/1999GL011293.
Schmidt, M. E., and A. L. Grunder (2009), The evolution of
North Sister: A volcano shaped by extension and ice in the
central Oregon Cascade Arc, Geol. Soc. Am. Bull., 121,
643–662, doi:10.1130/B26442.1.
Scott, W. E. (1987), Holocene rhyodacite eruptions on the
flanks of South Sister volcano, Oregon, in The Emplacement
of Silicic Domes and Lava Flows, edited by J. H. Fink, Geol.
Soc. Am. Spec. Pap., 212, 35–53.
Scott, W. E., and C. A. Gardner (1990), Field trip guide to the
central Oregon High Cascades, Part 1: Mount Bachelor–
South Sister area, Oreg. Geol., 52(5), 99–114.
Scott, W. E., and C. A. Gardner (1992), Geologic map of the
Mount Bachelor volcanic chain and surrounding area, Cascade Range, Oregon, scale 1:50,000, U.S. Geol. Surv. Map.,
I-1967.
Scott, W. E., C. A. Gardner, and A. M. Sarna-Wojcicki (1989),
Guidebook for field trip to the Mount Bachelor-South Sister-
10.1029/2012GC004341
Bend area, central Oregon High Cascades, U.S. Geol. Surv.
Open File Rep., 89–645, 66 pp.
Scott, W. E., R. M. Iverson, S. P. Schilling, and B. J. Fisher
(2001), Volcano hazards in the Three Sisters Region, Oregon,
U.S. Geol. Surv. Open File Rep., 99–437, 14 pp.
Sherrod, D. R., and J. G. Smith (1990), Quaternary extrusion
rates of the Cascade Range, Northwestern United States
and Southern British Columbia, J. Geophys. Res., 95(B12),
19,465–19,474, doi:10.1029/JB095iB12p19465.
Stelten, M. E., and K. M. Cooper (2012), Constraints on the
nature of the subvolcanic reservoir at South Sister volcano,
Oregon from U-series dating combined with sub-crystal
trace-element analysis of plagioclase and zircon, Earth
Planet. Sci. Lett., 313–314, 1–11, doi:10.1016/j.epsl.2011.
10.035.
Taylor, E. M. (1978), Field geology of the SW quarter of the
Broken Top quadrangle, Oregon, Spec. Pap. Oreg. Dep.
Geol. Miner. Ind., 2, 1–50.
Taylor, E. M. (1987), Field geology of the northwestern quarter
of the Broken Top 15 minute quadrangle Deschutes County,
Oregon, Spec. Pap. Oreg. Dep. Geol. Miner. Ind., 21, 1–20.
Trasatti, E., M. Bonafede, C. Ferrari, C. Giunchi, and G. Berrino
(2011), On deformation sources in volcanic areas: Modeling
the Campi Flegrei (Italy) 1982–84 unrest, Earth Planet. Sci.
Lett., 306(3–4), 175–185, doi:10.1016/j.epsl.2011.03.033.
Ueda, H., M. Ohtake, and H. Sato (2003), Postseismic crustal
deformation following the 1993 Hokkaido Nansei-oki earthquake, northern Japan: Evidence for a low-viscosity zone in
the uppermost mantle, J. Geophys. Res., 108(B3), 2151,
doi:10.1029/2002JB002067.
Wdowinski, S., and G. J. Axen (1992), Isostatic rebound due to
tectonic denudation: A viscous flow model of a layered lithosphere, Tectonics, 11(2), 303–315, doi:10.1029/91TC02341.
Weaver, C. S., and C. Michaelson (1985), Seismicity and volcanism in the Pacific Northwest: Evidence for the segmentation of the Juan De Fuca Plate, Geophys. Res. Lett., 12(4),
215–218, doi:10.1029/GL012i004p00215.
Wicks, C. W., D. Dzurisin, S. Ingebritsen, W. Thatcher, Z. Lu,
and J. Iverson (2002), Magmatic activity beneath the quiescent Three Sisters volcanic center, central Oregon Cascade
Range, USA, Geophys. Res. Lett., 29(7), 1122, doi:10.1029/
2001GL014205.
Wozniak, K. C. (1982), Geology of the northern part of
the southeast Three Sisters quadrangle, MS thesis, Dep. of
Geosci. Oreg. State Univ., Corvallis.
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