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






Methods

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








Conclusions



Acknowledgments


This poster is on line at: http://crack.seismo.unr.edu/ftp/pub/louie/dome/95agu/95agu.html