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REPORT
Catastrophic Mega-Scale Landslide Failure of
Large Volcanic Fields
16–22 September 2017
Cedar City and Bryce Canyon City, Utah, USA
CONVENERS
Robert F. Biek, Utah Geological Survey, Salt Lake City,
Utah, USA
David B. Hacker, Department of Geology, Kent State University,
Kent, Ohio, USA
Peter D. Rowley, Geologic Mapping Inc., New Harmony,
Utah, USA
FIELD TRIPS AND ANALYSIS
This six-day Thompson Field Forum investigated the concept of
exceptionally large catastrophic collapse of volcanic fields using
the distinguishing features of the newly discovered Markagunt
(MGS) and Sevier (SGS) gravity slides, Marysvale volcanic field,
southwest Utah, USA. Large landslides are known from hundreds
of volcanoes and increasing numbers of laccoliths around the
world, yet the mega-scale failure of volcanic fields themselves,
producing gravity slide structures so large that they blur the
boundary between gravitational and tectonic processes, has not
received widespread attention. This was a transformative week of
(1) redefining the physical limits of what is possible; (2) investi- Field Guide series.
gating evidence—on the outcrop—to identify unique characteris-
tics of structures and deposits indicative of mega-slide deforma-
tion and of fast versus slow processes; and (3) planning for future
collaborative studies.
First, some background. The MGS and SGS exhibit the full
range of structural features commonly seen in modern landslides,
but on a gigantic scale; they are among Earth’s largest terrestrial
landslides. The MGS remained undiscovered for so long precisely
because of its gigantic size (>5000 km2, >95 km long, >35 km
runout, estimated volume 3000 km3 [dimensions revised from
D.B. Hacker et al., 2014, Geology, v. 42, no. 11, p. 943–946]) and
its initially confusing mix of extensional, translational, and com-
pressional structures overprinted by post-MGS basin-range tecto-
nism. In comparison, the 1980 Mount St. Helens debris avalanche
was only about 3 km3. Preliminary mapping of the SGS, discov-
ered in 2015, shows it to be smaller (likely at least 1500 km2 in
size) and slightly older than the MGS. Both slides are composed
of large contiguous sheets of andesitic lava flows, volcaniclastic
rocks, and intertonguing ash-flow tuffs that record southward
catastrophic transport of the southern flank of the Oligocene to
30 GSA Today | December 2017
Miocene Marysvale volcanic field, preceded by gravitational
spreading on the Paunsaugunt thrust fault system. MGS emplace-
ment is currently constrained between 22 to 21 Ma, whereas SGS
emplacement is between 25 to 23 Ma. That features as large as the
MGS and SGS can remain undetected despite decades of geologic
mapping and research in a well-exposed area suggests to us that
other volcanic fields around the world may hold evidence of as-
yet-undiscovered exceptionally large gravity slides.
This field forum brought together 27 participants, including
nine students, who represented disciplines including rock
mechanics, landslides, volcanology, petrography, paleomagne-
tism, sedimentology, isotope geochemistry, structure, and tec-
tonics. We began the forum with a half-day meeting to introduce
the MGS and SGS and hear participant research presentations on
their relevant interests, then spent five days viewing and dis-
cussing exceptionally instructive outcrops. The meeting con-
cluded with a session that summarized the group’s findings and
planned for collaborative research, student involvement, and
funding opportunities. In early 2018, the conveners will submit a
field guide and participant abstracts for publication in the GSA
Using the MGS and SGS as examples, the conference empha-
sized the following:
• Different lines of evidence diagnostic of large to small cata-
strophic gravity slides;
• Cross-disciplinary assessment of extreme deformation recorded
by basal layers and associated injectites (clastic dikes), ultra-
cataclasite shear zones, pseudotachylyte, fragmented clasts, and
main and sidewall breakaways;
• Fabrics and rock types developed by rheomorphic versus tec-
tonic versus debris-avalanche modes of emplacement;
• Factors contributing to volcanic landslide initiation and trans-
port—why and how did these slides happen?;
• The role of magmatic intrusions in inflation of volcanic fields
and slope destabilization;
• Gravitational basement spreading of volcanic fields prior to
catastrophic failure;