1997 Fall AGU - Reflection Imaging of Upheaval Dome Impact

Seismic Reflection Imaging of Impact-Induced Faulting and Deformation at Upheaval Dome, Canyonlands National Park, Utah

J. N. Louie, Z. Kanbur, S. Chavez-Perez, and G. Plank

Seismological Lab (174), University of Nevada, Reno

(702) 784-4219 louie@seismo.unr.edu
http://www.seismo.unr.edu/ftp/pub/louie/dome/index.html

Abstract

Seismic imaging techniques applied to a reflection survey show two phases of faulting and deformation beneath Upheaval Dome, Utah. Recent work suggests Upheaval Dome may be the largest impact structure on the Colorado Plateau, having a ring syncline ~3 km in diameter. To better describe the impact deformation of a brittle layer above a viscous layer, as exists at Upheaval Dome with the underlying Paradox Salt, four institutions conducted a NASA-funded seismic reflection survey in January 1995. We obtained a 5 km section extending radially from the Dome's central depression using a 320 kg weight-drop source and a 48-channel off-end receiver spread 0.5 km long. The data show clear reflections as deep as 1.5 km. Imaging of the reflection section with velocity filtering and 3-d prestack Kirchhoff migration techniques reveals the geometries of deformed stratigraphy from the surface to the top of the Paradox Formation at 1.2 km depth. Stratigraphic terminations and fault-plane images show the paths of listric faults. We tie our sections to two well logs, one in the ring syncline and one outside the zone of deformation. Deformation appears in two phases with respect to depth, with listric normal faults in the ring syncline and the megablock zone confined to the part of the section above the Hermosa Formation. Listric faults flatten and sole into the clastic formations above the calcareous layers of the Hermosa at 1.0 km depth. At the base of the Hermosa, on the axis of the ring syncline, the Paradox has forced the Hermosa 0.1 km up and broken it with thrust faults. Post-impact relaxation of the crater form may have driven this deeper uplift. These two modes of deformation, driven from above and below, leave the upper Hermosa as the least-deformed part of the structure.

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

A 48-channel reflection survey collected 10 m station and 40 m source intervals along a 5 km profile both transverse and radial to Upheaval Dome.
Univ. of Utah's 700-lb hammer source provided hits for the reflection survey, and for refraction records across the center of Upheaval Dome by PASSCAL RefTeks ( reported here 2 years ago).
Dip-filtered reflection records show clear returns as deep as the base of the Paradox Fm. at 2.5 km.
3-D pre-stack Kirchhoff migration aided imaging of faults and faulted stratigraphy.

Results

Reflector depths in the pre-stack migrations tie very well to an acoustic log from the Ring Syncline, and a gamma log away from the structure.
Section 1 shows listric faulting of the Central Uplift, and little deformation of the top of the Paradox salt.
Section 2 shows the normal faulting outside the Ring Syncline, thinning the stratigraphy above the Hermosa Fm. in agreement with the logs.
Stratigraphic uplift observed in the center of Upheaval Dome cannot arise from Paradox salt diapirism. Radial listric faulting into a transient cavity provides the uplift.
The top of the Paradox is deformed up to 100 m vertically, but the top of the Hermosa appears undeformed.

Conclusions

There is no evidence of any salt diapir below the center of Upheaval Dome.
Deformation was driven from above by the impact, as shown by listric faulting above the Hermosa Fm.
Subsequent gravitational relaxation of the crater form fractured the Hermosa below the Ring Syncline and allowed some salt flow in the Paradox.

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.
Jerry Schuster of the Univ. of Utah provided the 700 lb hammer source as well as the 48-channel reflection recording system.
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.