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GSA TODAY | NOVEMBER 2016 Catastrophic Mega-Scale
Landslide Failure of Large
Volcanic Fields
Cedar City and Bryce Canyon City, Utah, USA
16–22 September 2017
www.geosociety.org/fieldforums/
CONVENERS
Robert F. Biek, Utah Geological Survey, Salt Lake City, Utah,
USA, bobbiek@utah.gov
David B. Hacker, Department of Geology, Kent State University,
Kent, Ohio, USA, dhacker@kent.edu
Peter D. Rowley, Geologic Mapping Inc., New Harmony, Utah,
USA, pdrowley@rushisp.com
DESCRIPTION AND OBJECTIVES
This six-day field forum is designed to investigate the concept
of exceptionally large catastrophic collapse of volcanic fields using ment event. The MGS preserves the first reported occurrence of
the distinguishing characteristics and geologic implications of the
gigantic Markagunt gravity slide and Marysvale volcanic field,
southwest Utah, USA.
Growing evidence of large landslides on volcanic edifices
during the past few decades has led to their identification and
study at hundreds of volcanoes around the world. Such flank fail-
ures and sector collapses are a common form of volcano insta-
bility and a significant part of a volcano’s development. However,
mega-scale failure of volcanic fields, producing gravity slide struc- of the field may have tilted strata on the southern flank gently
tures so large that they blur the boundary between gravitational
and tectonic processes, has not been widely recognized.
The newly discovered Miocene Markagunt gravity slide (MGS)
in southwestern Utah, USA, provides an ideal setting for investi-
gating the structure and evolution of mega-scale collapse features
of volcanic fields. Superb exposures of internal deformation, from
source-area breakaway faults to distal debris-avalanche deposits,
show that the MGS exhibits the full range of structural features
commonly seen in modern landslides, but on a mega-scale. The
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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 Geology, v. 42, no. 11,
p. 943–946) and initially confusing mix of extensional, transla-
tional, and compressional structures overprinted by post-MGS
basin-range tectonism. That features as large as the MGS can remain
undetected despite decades of geologic mapping and research in
the area suggests to us that other volcanic fields around the world
may hold evidence of as-yet-undiscovered, exceptionally large
gravity slides. Furthermore, some modern volcanic fields may
possess the conditions capable of generating similar large slides.
Only the Eocene Heart Mountain gravity slide (HMGS) in
Wyoming, USA, is a terrestrial slide of comparable size; it was
considered unique until discovery of the MGS.
The MGS represents southward catastrophic failure ca. 22–21 Ma
of the southwestern sector of the Oligocene to Miocene Marysvale
volcanic field. Recent geologic mapping indicates that the MGS is
a large contiguous volcanic sheet of allochthonous andesitic
mudflow breccias and lava flows, volcaniclastic rocks, source
intrusions, and intertonguing regional ignimbrites. The presence
of basal cataclastic breccias, clastic dikes, and pseudotachylyte, the
uniformity of kinematic indicators, and the overall geometry of
the MGS show that it represents a single catastrophic emplace-
landslide-generated pseudotachylyte in North America, among
only a handful known throughout the world. Catastrophic failure
was preceded by gravitational spreading of the volcanic field, well
displayed along the Rubys Inn thrust fault zone in and near Bryce
Canyon National Park. Continued late-stage growth of the
Marysvale volcanic field, which loaded ever more volcanic rocks
on a structurally weak basement of ash-rich volcaniclastic strata,
created conditions necessary for gravity sliding. Inflation of the
volcanic pile by intrusions above the batholith beneath the heart
southward, providing tilted planes for sliding on the underlying
weak volcaniclastic strata.
The MGS is significant because it provides a stunning example
of gravity-slide structures so large that they may be mistaken for
tectonic features. Some low-angle normal faults, including some
interpreted as detachment faults, may need reassessment in light
of the implications of the MGS. For example, gravitationally
driven, unrooted low-angle normal faults that emplace younger
rocks on older rocks, called attenuation or denudation faults, are
common in the Great Basin. They represent failure of upper parts
of ranges along regional shale units following uplift along range-
front normal faults and are similar in many respects to the MGS.
This Thompson Field Forum will Emphasize:
• Different lines of evidence diagnostic of large to small cata-
strophic gravity slides, using the MGS as an example;
• Cross-disciplinary assessment of extreme deformation
processes recorded by basal and lateral breccias and associated
clastic dikes, ultracataclasite shear zones, jigsaw-puzzle frac-
turing of clasts, striations and grooves, riedel shears, breakaway
and internal faults, and pseudotachylyte;
• Factors contributing to volcanic landslide initiation and
transport;