Impact Deformation at Upheaval Dome, Canyonlands
National Park, Utah, Revealed by Seismic Profiles
J N Louie, S Chavez-Perez, and G Plank
Seismological Lab 174, Univ. of Nevada, Reno, NV 89557-0141;
702-784-4219; louie@seismo.unr.edu
Abstract
Seismic refraction and reflection surveys show structures
characteristic of meteor impact and associated salt
deformation beneath Upheaval Dome, Utah.
Geologists previously thought this radially-symmetric
structure to be due to salt diapirism in the underlying
Paradox Formation.
However, recent work suggests Upheaval Dome
may be the largest exposed impact structure on the Colorado
Plateau.
To test this hypothesis JPL, the Univ. of Nevada, Reno,
the Univ. of Utah, and CSU Dominguez Hills conducted
NASA-funded geophysical surveys across Upheaval Dome
in January 1995.
Other participants will report gravity and geologic
mapping results.
We obtained a reversed refraction profile across the
center of Upheaval Dome, and a 5 km reflection profile
extending radially outward.
The refraction data are remarkably good, and we
can pick first arrivals to 30 ms or better accuracy.
Raw reflection records also clearly show structures
down to the top of the Paradox.
Analysis of refraction picks by forward-modeling and
tomographic inversion shows no evidence of any salt
diapir within 500 m below the Dome's central depression.
Instead, velocities are relatively low below the center
of the structure, with refractions clearly showing
high-velocity bodies above the Paradox and below or
outside the ring syncline.
The high velocities may be due to secondary cementation
of limestones above the Paradox, closing open fractures
created by impact in a 4 km diameter ring, below the
ring syncline.
Any uplifted salt forming the high velocities could result
from impact-generated flow radially outward of the Paradox
salt, or from viscous relaxation of the crater form.
Motivation
- Upheaval Dome may be the eroded remnant of the largest meteor impact to
the Colorado Plateau.
- The structure has long been thought of as a salt dome, but may be a rare
terrestrial example of cratering above a deformable layer.
- Seismic profiling was needed to constrain the presence of any salt
diapir, and characterize deformation within and above the Paradox Formation.
Methods
- Univ. of Utah's 700-lb hammer source provided thousands of hits recorded
across the center of Upheaval Dome by PASSCAL RefTeks with high-frequency
geophone strings.
- RefTek GPS clocks allowed synchronized picking of all hits, and stacking
at each receiver channel.
- A 48-channel reflection survey collected 10 m station and 40 m source
intervals along a 5 km profile radial to Upheaval Dome.
- Both forward modeling and wide-ray tomography of refraction arrivals use
a finite-difference travel time code to accurately account for lateral
velocity variations and the extreme topography.
We used a 700-lb enhanced weight drop hammer, the Bison EWG
owned by Jerry Schuster's group at the University of Utah, as
the source for both the refraction and reflection experiments.
Source points could only be located around the periphery of Upheaval
Dome, at points of road access.
With the area's extreme topography, helicopter support allowed placement of
portable RefTek recorders in a refraction profile crossing the center of
the structure.
(Marcos Alvarez of IRIS in Upheaval Dome.)
The RefTeks recorded continuously for two and a half days, while the hammer
source was placed at five different source points. At each point, the source
was activated to hit the ground between 200 and 2000 times.
All RefTeks had GPS clocks, and we picked source hit times from a recorder
at each source point. We could then stack trace rcords from each RefTek
channel for each source location.
Stacked record from the near off-end source point at the Upheaval Dome
Picnic Area, overlooking the central depression from its southeast side.
Stacked record from the far off-end source point at the Wilhite Trailhead,
about 5 km southeast of Upheaval Dome.
Stacked record from the far reversal source location on Horsethief Point,
about 10 km northwest of Upheaval Dome.
Lateral continuity of these data are surprisingly good.
Stacked record from the very far off-end Willow Flat source point, about
10 km southeast of Upheaval Dome. Only closer receivers in the central
depression yielded pickable records at this far distance.
Stacked record from the distant fan source point at the Island in the Sky
Visitor Center, about 25 km northeast of Upheaval Dome. No arrivals
are apparent in this case after stacking.
Results
- Clear first arrivals were picked from stacked refraction receiver gathers
at distances up to 15 km.
- Three in-line source locations crossing the center of Upheaval Dome
produced good picks on a reversed refraction profile.
- Longer-offset picks show the top of the Paradox Formation as a
high-velocity refractor, faster than the overlying indurated sandstone with
interbedded limestones.
- Refraction rays passing within 500 m below the center of Upheaval Dome
show no evidence of early arrivals.
- Refraction rays passing below Buck Mesa and Syncline Valley have very
early arrivals.
- Reflection, gravity, and geologic analyses will be presented at a later
date.
Conclusions
- There is no evidence of any salt diapir below the center of Upheaval
Dome.
- High velocities at depth ringing Upheaval Dome may be due to: (1) an
asymmetric bulge of the top of the Paradox; or (2) to the presence of a
relatively low-velocity shattered zone at the center of the structure.
- A central shattered zone is more consistent with the small deformation of
stratigraphy observed on Buck Mesa.
- Any salt uplift may be located under the bottoming points of the ringing
listric faults, and may contribute to central peak formation.
Acknowledgments
- This project was funded by the NASA Planetary Geology Program, and is a
collaboration with Jeff Plescia and Ken Herkenhoff of the Jet Propulsion
Laboratory, and Brian Kriens of C.S.U. Dominguez Hills.
- Eugene Shoemaker originally recognized the impact origin of Upheaval
Dome.
- The staff of Canyonlands National Park helped us to mitigate any
environmental impacts of the geophysical surveys.
- The Incorporated Research Institutions for Seismology provided the
PASSCAL recorders, and allowed crucial participation in the field by Marcos
Alvarez of the Stanford Instrument Center.
- Jerry Schuster of the Univ. of Utah provided the 700 lb hammer source as
well as the 48-channel reflection recording system.
This poster is on line at:
http://www.seismo.unr.edu/ftp/pub/louie/dome/95agu/95agu.html