Vanessa Swenton Sigma Xi 2021 Research Showcase Data in this presentation is property of Vanessa Swenton and Martin Streck of Portland State University, Portland, Oregon, USA.

1. Assessing the space
and time trends in
Miocene co-CRBG
rhyolites and rhyolites
of the High Lava Plains
trend in eastern
Oregon
SIGMA XI RESEARCH PRESENTATION 2021
VANESSA SWENTON

2. Miocene rhyolite
volcanism of the
Pacific NW can be
summarized as
occurring in 3 primary
volcanic fields:
1) co-Columbia River
Basalt Group
(CRBG) rhyolites
2) Rhyolites of the
Yellowstone-Snake
River Plain (YSRP)
track
3) Rhyolites of the
High Lava Plains
(HLP)
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3. co-CRBG rhyolites
~17.3–15 Ma
Y-SRP rhyolites
14 Ma to present
co-CRBG rhyolites
are a result
ponding and
radially spreading
of rising mantle
plume material
NE-younging YSRP
rhyolites are a
product of the
North American
plate migrating
over the stationary
Yellowstone plume
tail
2

4. co-CRBG rhyolites
~17.3–15 Ma
HLP rhyolites
<12 Ma
Numerous models
attempt to explain
the NW-younging
HLP rhyolites as being
a result of either:
Buoyancy-driven
westward plume
spreading
Or
Evolving dynamics of
the subducting
Cascadia slab (no
plume involvement)
Y-SRP rhyolites
14 Ma to present
3

5. Jordan et al. (2004)
Camp (2019); Camp and Wells (2021)
Examples of HLP Models that
Involve the Plume
Plume material ponds around
the craton boundary beneath
the North American plate
producing co-CRBG rhyolites
Buoyancy-driven westward
spreading of plume material melts
the crust, producing HLP rhyolites
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6. Examples of HLP Models that
do Not Involve the Plume
Ford et al. (2013)
Hawley and Allen (2019)
Mantle convection is induced by
slab rollback and trench
migration, which heats the crust
to produce rhyolites
Mantle rises buoyantly through a
westward propagating slab tear,
where it then melts the crust to
product HLP rhyolites
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7. A lack of regional
~15–12 Ma
rhyolites (an
apparent ~3 m.y.
eruptive hiatus)
and their limited
spatial overlap led
previous
researchers to
conclude HLP and
co-CRBG rhyolite
provinces were a
result of separate
and distinct
magmatic and
tectonic processes
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co-CRBG rhyolites
~17.3–15 Ma
HLP rhyolites
<12 Ma

8. Compilation mapping from Ferns et al. (1993a; 1993b), Greene et al. (1972), Webb et al. (2018)
However, this interpretation was developed with an incomplete
dataset, as ~33 of the total ~50 rhyolite eruptive centers in the area
where the 2 provinces overlap had yet to be dated.
Thus, the
relationship of
the two
provinces
remained
unclear.
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9. 8
Geochemical data (XRF) from previous
studies* reveals the significant compositional
variability among regional rhyolites.
There does not appear to be a strong correlation
between age and composition among regional
rhyolites.
With a limited geochemical and geochronological
dataset, it is uncertain whether there is a definitive
distinction between older, co-CRBG rhyolites and
younger rhyolites of the HLP.
*Compilation of XRF data from numerous previous studies
Blue symbols = >15 Ma
Yellow symbols = <12 Ma
Red symbols = Precise age unknown
All centers are rhyolite lavas unless otherwise noted in legend.

10. Our goal was to fill critical gaps in the regional
comprehensive space-time record in order to refine current
models or generate a new model that accurately describes
the processes that generated the HLP volcanic trend and
the relationship of the HLP and co-CRBG provinces.
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11. The Yellowstone plume is most likely involved
in CRBG volcanism, Earth’s youngest flood
basalt province.
Understanding the magmatic and tectonic
processes involved in generating co-CRBG and
HLP rhyolites has the potential to refine our
understanding of the dynamics of mantle
plume spreading and plume-lithosphere
interactions, a process that impacts our planet
at a global scale.
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12. Whole rock geochemical signatures
via XRF and ICP-MS at WSU**
SEM and Petrography at PSU**
Precise 40Ar/39Ar dating of sanidine phenocrysts at OSU
Methods
These data on 26 previously unanalyzed
rhyolites* allowed us to correlate or distinguish
rhyolites and complete the comprehensive
space-time record of regional rhyolites.
*Ages are pending for 3 additional rhyolites
**Geochemical data is a component of this chapter of my dissertation research
but is the primary focus of a separate publication and is not discussed in this
presentation.
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13. A Previously Unrecognized Periodicity in Regional Miocene Rhyolite Volcanism
Compilation of ages from this study and numerous previous studies
16.3–14.4 Ma
Associated with CRBG eruptions
12.1–9.6 Ma
Emplaced primarily where HLP province
and co-CRBG rhyolites overlap
7.3–5.2 Ma
In the central and westernmost HLP
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Cumulative Age Distribution of 100 Distinct Regional Silicic Centers
Relatively gradual onset period starting at ~17.5 Ma
Relatively gradual onset period starting at ~9.0 Ma
Addition of our new ages reveals 3 distinct
pulses of rhyolite volcanism

14. A Previously Unrecognized Periodicity in Regional Miocene Rhyolite Volcanism
Compilation of ages from this study and
numerous previous studies
16.3–14.4 Ma
Peak eruptive activity began
with the 16.29 Ma* Beulah
Reservoir rhyolite and
waned after the 14.37 Ma*
Strawberry Rhyolite
12.1–9.6 Ma
Initiated with
eruption of the
12.05 Ma* Beaty’s
Butte rhyolite and
waned with eruption
of the 9.63 Ma*
Devine Canyon Tuff
7.3–5.2 Ma
Peak eruptive activity
began with eruption of the
7.28 Ma* ‘Horse
Mountain, north’ rhyolite
and waned after eruption
of the 5.17 Ma* Bug Butte
top rhyolite
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We refined the previously observed ~16.5–15
Ma eruptive episode to 16.3–14.4 Ma and
~15–12 Ma eruptive hiatus to 14.4–12.1 Ma
~2.3 m.y.
eruptive
hiatus
~0.6 m.y.
eruptive
hiatus
*age from a previous study

15. Observed lack of an expected pulse of mafic volcanism ~13–11 Ma
Compilation of ages from this study and numerous previous studies
Regional mafic volcanism
Bar height (y-axis) of no assigned value
Voluminous
outpouring of CRBG
flood basalts
Dispersed eruptions of the Owyhee
Basalts, Tims Peak basalt, and
Keeney Sequence
Episodes of basalt volcanism within the central and
western HLP
Unlike the 16.3–14.4 Ma rhyolite episode, there is no prominent pulse of
mafic magmatism immediately prior to onset of 12.1–9.6 Ma rhyolites
(Bin width of 0.5 m.y.)
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16. New Regional
Miocene Rhyolite
Eruptive Center
Compilation Map
Our new ages conform
to the previously
known NW-younging
HLP trend.
Most of our new ages
are within the 12.1–9.6
Ma episode and lie
within and just outside
of the easternmost
extent of the HLP
province.
Sampling area for
this study
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Compilation of mapping from numerous
sources. Compilation of ages from this
study and numerous previous studies.

17. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
A new model is needed to address:
1) The ~2.3 m.y. hiatus in rhyolite eruptions
between ~14.4 Ma and 12.1 Ma
2) The strong recommencement of rhyolite
eruptive activity at ~12.1 Ma
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18. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
Cascadia slab rollback is unlikely the primary
driving force behind the HLP volcanic trend.
Recent seismic studies reveal a highly fragmented slab
beneath North America (e.g., Long, 2016), and describe
fragmentation of the slab and resulting cold anomalies
(Zhou et al., 2018) or a westward-propagating tear in
the slab as the driving force for mantle convection
(Hawley and Allen, 2019).
Most importantly, slab rollback is a phenomena
that is assumed to result in continuous mantle
flux to the crust, and the newly revealed episodic
natural of rhyolite volcanism indicates that this is
not the case.
Hawley and Allen (2019)
Slab fragmentation reduces the plausibility of models
where slab subduction or rollback is the sole or primary
driving mechanism for westward-trending crustal melting
(Long et al., 2009; Ford et al., 2013; Wells and McCaffrey,
2013).
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19. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
The 14.4–12.1 Ma rhyolite eruptive hiatus
and lack of ~13–11 Ma mafic eruptions may
be correlated to decreased plume flux post-
15 Ma
Migration of the North American craton over the plume
tail at ~15 Ma resulted in the once rapid radial spreading
of plume material to transition into the narrow age-
progressive rhyolite volcanism of the YSRP track to the
east and relatively slow westward spreading initiating in
areas west of the craton boundary (e.g., Camp, 2019;
Camp and Wells, 2021).
Jordan et al. (2004)
Decreased plume flux would decrease basaltic volcanism
and associated rhyolite volcanism, as observed in the
data.
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20. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
Lack of ~13–11 Ma basalts could also be
attributed to hinderance by regional crust.
Older, widespread, erupted and non-erupted co-CRBG
rhyolites distributed throughout slightly thicker
regional crust of eastern Oregon may have prohibited
basaltic magmas from ascending through the crust to
the surface (Ford and Grunder, 2011; Streck et al.,
2015; Townsend, 2019)
Eastern and central Oregon have been subject to
erosion via Basin and Range extensional faulting over
the past ~10 Ma.
There is a notable lack of precise age and geochemical
data for some exposed <15 Ma regional basalts, some
of which are unnamed, and some are postulated to be
associated with the ~7 Ma Drinkwater Basalt (Greene
et al., 1972).
Lack of ~13–11 Ma basalts does not
necessarily negate their existence.
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21. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
Our compilation of ages from previous studies found
the third and most recent pulse of rhyolite eruptive
activity to be ~9.0–5.2 Ma, which confirms findings
from previous researchers.
These studies describe the relatively larger distribution area and
lesser frequency of rhyolite eruptions <10 Ma to be a result of the
greater areal distribution of mafic input as it spread westward and
eventually buttressed against eastern side of the thick lithosphere
beneath the Cascade Arc (e.g., Ford et al., 2013)
Enough heated mantle was present to generate the 9.0–5.2 Ma
rhyolites and coeval basalts of the HLP but waned after the material
likely began to cool with increased spreading.
Camp (2019)
Imaging of the low-density anomaly at 75 km depth interpreted as
westward spreading plume that can be seen ponding beneath the
western HLP, just east of the Oregon Cascades.
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22. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
Pre-existing models of channelized
westward plume spreading fit, but not
entirely with respect to timing.
Camp (2019); Camp and Wells (2021)
Camp and Wells (2021) model where westward
migration of plume material did not occur until the
tail was well established beneath the craton (~12
Ma) would explain a hiatus in rhyolite volcanism in
eastern Oregon.
However, if westward spreading began at ~12 Ma
when the plume tail was as far east as southcentral
Idaho, it would take a substantial amount of time to
migrate to the area where 12.1–9.9 Ma rhyolites
erupted in eastern Oregon.
In addition, these models do not reconcile the significantly
wide distribution of 17.5-14.4 Ma rhyolites, and they do
not address the strong recommencement and
concentrated area of ~12.1-9.6 Ma rhyolite eruptions in
the study area.
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23. A New Model for Miocene Rhyolite Volcanism of eastern Oregon
At this point in the study, regional fault
systems including the Brothers Fault Zone
(BFZ), Northwest Basin and Range
(NWBR), and a relatively unstudied area
of NNW-trending faults north of Steens
Mountain (SM) appear to be a relevant
piece of the volcanic story and require
further investigation.
Trench et al. (2012)
NWBR extension and rotation of the
Pacific Northwest significantly effect
the region by providing crustal
weaknesses for magmas to
propagate to the surface and can
result in erosion and burial of
volcanic deposits.
Though slab dynamics alone are unlikely to be the driving force
behind HLP rhyolite volcanism, the combination of effects of
rollback, subduction counterflow, NWBR extension, and rotation of
the PNW should be included in a model for the HLP province.
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24. Conclusions
Decreased mantle flux is likely the
primary mechanism behind the lack of
basaltic volcanism immediately prior to
the 12.1–10.1 Ma rhyolite episode, with
lesser but still notable effects from
hinderance by regional crust, erosion,
and lack of data on regional basalts.
Decreased mantle flux
would result in the lack of
rhyolite eruptions from
14.4 Ma to 12.1 Ma.
Miocene rhyolite volcanism of
the HLP and CRBG provinces in
eastern Oregon occurred in a
step function periodicity, with
three prominent episodes and
two distinct eruptive hiatuses.
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25. Conclusions
Interpretation of our results is still
ongoing, but we propose a model where
Yellowstone plume spreading was
widespread and likely channelized as early
as ~17.5 Ma, resulting in widespread
~17.5-14.4 Ma rhyolites.
NWBR faulting, PNW rotation, and the
Brothers Fault Zone display how the crust
needed to accommodate significant
extension ~17-10 Ma, which would have
eventually thinned the regional lithosphere
and created crustal weaknesses for basalts to
begin infiltrating the crust again.
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Either deep plume material itself
or upper mantle heated by the
plume resided in eastern Oregon
as North American migrated over
the tail, but mantle flux
decreased enough to cease
rhyolite eruptions ~14.4-12.1 Ma.

26. Conclusions
The Yellowstone plume is the
likely primary driving
mechanism behind co-CRBG
volcanism. The 12.1-9.6 Ma
episode is a combined effect of
the Yellowstone plume, heated
upper mantle, and extension
of the lithosphere. The 7.3-5.2
Ma episode is primarily a
result of heated upper mantle
and tectonic processes, with
less effect from the
Yellowstone plume.
This model involves interpreting each rhyolite
eruptive episode as separate and distinct,
where pulses are explained primarily by
punctuated mafic inputs into the crust, rather
than models with continuous spreading of
heated mantle, where distinct periods of
eruptive activity are difficult to reconcile.
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27. Alisa Humphrey
Co-authors
Advisor: Martin Streck
Dan Miggins
Bill McIntosh
PhD Committee
John Bershaw
Anita Grunder
Jon Fink
Dave Stuart
Thank you
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