Strain for Arizona and the Colorado Plateau Determined from Campaign and Continuous GPS Velocities
The content contained here is a simple reproduction of content presented at the Seismological Society of America Meeting, Spring 2009. Please use the following reference for any citations.
Holland, A.H. and R.A. Bennett, 2009, Strain for Arizona and the Colorado Plateau Determined from Campaign and Continuous GPS Velocities, Seismological Research Letters Volume 80, No. 2, Abstract on page 379.
Abstract
We determined a regional strain field for the Colorado Plateau and surrounding region from GPS velocities. There is a distinct extensional strain concentrated along the western and southern boundary of the plateau. While the number of continuous GPS stations in Arizona and surrounding regions are growing the station spacing is generally quite large. We conducted campaign measurements of benchmarks in Arizona that had been previously measured by the NGS. The previous GPS measurements conducted by the NGS were part of the Federal Base Network (FBN) and consist of a several separate observations in 1998 and 1999. These observations generally consisted of four to five hour observation periods. Some of these FBN benchmarks were re-observed in 2008 with longer observation periods. The data were analyzed using GAMIT/GLOBK version 10.3 to obtain velocities for these and the continuous sites operating in the region. The strain field obtained from GPS velocities indicates that there is primarily east-west oriented extension concentrated along the boundary of the Colorado Plateau.
Introduction
Figure 1 - Red circles are earthquakes obtained from the USGS NEIC PDE catalog from 1973 through 2008. Purple are earthquakes from the Arizona Earthquake Information Center at Northern Arizona University from 1830 to 1973. Most of these have fairly large location errors. The yellow star is the May 3, 1887 M 7.4 Pitaycachi, Mexico earthquake, which is a normal fault with up to 4 m of offset (Natali & Sbar, 1982; Wells & Coppersmith, 1994). Geological Provinces are outlined in white and labeled. RGR is the Rio Grande Rift. The white dashed line is the Pacific-North America plate boundary as defined by Bird (2002). The velocity shown for the Pacific plate was calculated for relative to North America from NUVEL1A (DeMets et al., 1994). The dark gray lines are Quaternary faults from USGS geologic maps (http://tin.er.usgs.gov/geology/state).
The Basin and Range (B&R) is characterized by a roughly east-west Cenozoic extension with a topographic expression of normal faults perpendicular to this sense of motion with thinned continental crust. The Southern B&R is characterized by thin crust and low elevations while the Northern B&R also has thin crust but higher average elevations. The Colorado Plateau (CP) is a fairly undeformed high elevation region with thicker than average continental crust. The Transition Zone is the region in which the transition from thin B&R to thick CP crust occurs.
Extension within the SB&R is thought to have ceased by about 11 Ma. Early extension has a strong spatial and temporal correlation with basalt magmatism supporting increasing heat flows associated with the removal of the Farallon slab at about 25 Ma. The timing of extension within the SB&R appears to be consistent with gravitation collapse (Spencer et al., 1995). Gravitational collapse due to potential energy gradients may drive a significant amount of extension within the B&R as well as forces from the Pacific-North America plate Boundary (Humphreys and Coblentz, 2007; Flesch et al., 2000; Sonders and Jones, 1999)
Arizona Campaign GPS
Campaign GPS measurements were conducted in 2008 and 2009 by re-observing a small subset of National Geodetic Survey (NGS) Federal Base Network (FBN) benchmarks. These benchmarks were previously observed by the NGS as part of the FBN spatial reference control program, which provides a greater spatial density than is available for continuous GPS (CGPS) sites within Arizona. The previous FBN observations used in this study are from the fall of 1998 and spring of 1999. The data generally consist of two to several 5 to 6 hour observation periods for each site. The data from 2008 and 2009 contain at least two consecutive 24-hour observation periods. The data was processed using the GAMIT/GLOBK version 10.3 software (Herring et al. 2006a&b).
Figure 2 - Blue arrows indicate velocities for CGPS sites that have been more than 2 and ½ years of data. Yellow arrows are campaign GPS velocities. Velocities are relative to Stable North America Reference Frame (SNARF) (Blewitt et al., 2005) error ellipses are for the 2σ confidence interval. The green lines are two transects of topography and GPS velocities within 50 km of the line. FBN benchmarks that have not been re-observed as part of this study are shown as purple squares. White squares are FBN benchmarks which have been destroyed or re-occupation was not possible. Blue squares with no velocities are CGPS sites without 2 ½ years of observations or were too great to display at this scale (these are marked with a dark-blue dot in the center).
Figure 3 - Topographic and velocity profiles with 1σ error bars; changes in velocities indicate regions of strain. The northern transect shows that extensional strain is accumulating west of the Colorado Plateau beginning in the transition zone. The CP appears to be non-deforming, but may be moving away from eastern New Mexico. The largest gradient for the southern transect appears to be west of Tucson, which is approximately located at the cluster of velocity estimates.
Strain Field
| The strain field was modeled from horizontal velocities using SSPX (Cardozo and Almendinger, 2008). The strain field was calculated on a 100 km grid spacing and 100 km distance weighting. The large distance weighting causes the strain field to have a large spatial smearing that is more pronounced in areas of sparse GPS velocities. The largest errors in the strain field occurred near the boundaries and where GPS measurements were very sparse. A larger subset of GPS velocities than those shown in the maps were chosen to avoid the edge effects on the uncertainties for our region of interest. All grid points within our maps have reasonable errors and can be considered significant. |  |
Dilatation represents an increase in area. Positive dilatation indicates regions of extension and negative dilatation indicates regions of compression. The greatest amount of extension is occurring due to the opening of the Gulf of California. There is also a large dilatation on the north-western edge of the CP, which is coincident with a band of significant seismicity (Figure 1). Most of the Southern B&R and the transition zone appear to be extending, although this extension cannot be as well correlated to active seismicity.
Rotations about a vertical pole through the center of the grid point; red is clockwise and blue is counterclockwise rotation. Large clockwise rotation associated with the opening of the Gulf of California, which is causing a positive rotation within the Southern B&R. Rotation within the Colorado Plateau is generally small and consistent with the rotation loosely constrained by Bennett et al. (2003) for the N. CP.
Discussion
• The dilatation field shows a clear concentration in extension at the PA/NA plate boundary as well as the northwest boundary of the Colorado Plateau
o High dilatation rates correspond to increased seismic activity
• It appears that the southern Basin and Range is undergoing distributed deformation associated with the PA/NA plate boundary
o Can be seen in both the rotation and dilatation fields from modeled strain rates
• Dilatation within the southern Basin and Range while small is significant as modeled from available GPS velocities
o There are disproportionally too few mapped Quaternary faults and modern earthquakes to support strain rates through brittle faulting
• Is there another discrepancy between geologic and geodesic rates? Is it possible that strain rates are small enough that most of the extension is being accommodated aseismically through ductile flow in the lower crust, low angle lower-crustal detachment, or some other process?
Future Work
• We will be observing another 60 FBN benchmarks in May 2009 in collaboration with the Arizona Geological Survey.
o Allow a more focused study in areas of where additional seismic hazards would have significant impacts, as well as those areas that have indicated measurable strain in this study.
o Allow possible identification of zones or faults that are accumulating strain.
o Determine whether or not the Colorado Plateau is internally deforming and its modern rotation rate.
o Constrain the deformation pattern of the often-neglected Southern B&R.
• Explore the effects that Glacial Isostatic Adjustment (GIA) included in the SNARF reference frame may have on our strain field, by also determining a strain field from ITRF2005 velocities.
Acknowledgements
- Everyone in the UofA geodesy labratory for their helpful discussions and advice, especially Sigrún Hreinsdóttir for hours of tutoring on GPS processing
- National Geodetic Survey for supplying their data from previous surveys
- UNAVCO for providing the Trimble 4000 and Zephyr antennas which were used to collect data
- The John and Nancy Sumner Scholarship Fund which has provided most of the funding for this project to present
- The AZGS and the AISN project for supporting another campaign
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