A Fresh Look at Western Grand Canyon
Lava Dams: Faulting and Incision

Most people view Grand Canyon as an unchanging geological wonderland. Those of us who have been around the canyon for a while know that, in fact, it is a mosaic of the ancient and the active. Those two extremes in geologic stability collide in western Grand Canyon. The Hurricane and Toroweap faults are the most active faults in northwestern Arizona. They slice through what is often perceived as the oldest part of the Canyon, and this creates an opportunity for understanding the recent changes in the Big Ditch.
These normal faults, which trend north to south, cross the southwestward flowing Colorado River near river miles 179 and 191 (Figure 1a). The faults are downthrown to the west, meaning that the plateaus west of each of the faults are moving down relative to those on the eastside. The offset rates for each of the faults vary, but the Hurricane and Toroweap faults are moving vertically at roughly 260 and 360 feet per million years, respectively. Multiple landforms, such as alluvial fans, debris flows, lava flows, and volcanic vents—all younger than two million years—as well as Paleozoic rock layers have been ruptured by large-scale earthquakes on the faults (Jackson et al., 1990; Huntoon, 1977; Stenner et al., 2001; Fenton et al., 2001). It takes an earthquake roughly equal to or greater than seven on the Richter scale to cause these ruptures, which are up to ten feet high. Older landforms along each of the faults have more displacement than younger landforms because they have been around long enough to have experienced multiple earthquakes. So, although there is a combined total of 1900 feet of movement on these faults, it is not as if there is a big waterfall where the faults cross the river. Surface ruptures occur in increments of time, allowing the river to erode through each of these offsets. The Prospect debris fan (Lava Falls Rapid) is 3000 years old and is not ruptured by the Toroweap fault, which runs right through it. It has been at least 3000 years since the Colorado River has had to erode through a fault scarp there. Although the faults are still active, they are old; movement on this fault system started no later than 3.5 million years ago (Fenton et al., 2001; Billingsley and Workman, 2000).
What goes up must come down. When a fault scarp crosses a river channel, the river has a tendency to cut through it to return to its original bed elevation. Because plateaus on the east of each of these faults are moving up relative to those the west, it seems intuitive that the river will cut through the eastern plateaus faster than the western (Fenton et al., 2001; Pederson et al., 2002). The Colorado River has sufficient power to quickly downcut through material (Lucchitta et al., 2000) that is uplifted during individual fault movements of ten feet or less.
The exciting thing about this research is that it could explain some of the phenomenon that many of us have seen throughout our Grand Canyon careers. Western Grand Canyon appears “old-looking,” whereas, eastern Grand Canyon looks like it could have been cut not too long ago in the geologic past. The data collected by Lucchitta et al. (2000) and Pederson et al. (2002) support this perception by telling us that incision rates in eastern Grand Canyon are at least double those of western Grand Canyon. .

That the faults are the sole cause of this difference has been seriously questioned (Hanks and Blair, 2003), in part because of the mechanics of the faulting and in part because the river is not flowing on bedrock (see later article). A further consideration that we think is important is the presence of lava dams in western Grand Canyon throughout the past 600,000 years, the period over which these incision rates have been calculated. Some of these dams were stable (Figure 2); some of them failed catastrophically, but all of them had an effect on the Colorado River that likely retarded downcutting rates in western Grand Canyon. We’ve already alluded to lava-dam failures in previous articles, so in our next article, we’ll talk about the evidence for dam failure and dam stability.

Cassie Fenton & Bob Webb

References:
Billingsley, G.H., and Workman, J.B., 2000, Geologic map of the Littlefield 30’X 60’Quadrangle, Mohave County, northwestern Arizona: U.S. Geological Survey Geologic Investigations Series Map I-2628, 1 sheet, 25 p. + map scale 1:48 000.

Fenton, C.R., Webb, R.H., Pearthree, P.A., Cerling, T.E., and Poreda, R.J., 2001. Displacement rates on the Toroweap and Hurricane faults: Implications for Quaternary downcutting in Grand Canyon. Geology 29: 1,035–1,038.

Hanks, T.C. and Blair, J.L., 2003, Differential incision of the Grand Canyon related to Quaternary faulting—Constraints from U-series and Ar/Ar dating: Comment and Reply, Geology online forum @www.gsajournals.org, June 2003, p. e16–17.

Huntoon, P.W., 1977, Holocene faulting in the western Grand Canyon, Arizona, Geological Society of America Bulletin, v. 88, p. 1619–1622.

Jackson, G.W., 1990, Tectonic geomorphology of the Toroweap fault, western Grand Canyon, Arizona: Implications for transgression of faulting on the Colorado Plateau. Arizona Geological Survey Open-File Report 90-4, p. 1–66.
Lucchitta, I., G.H. Curtis, M.E. Davis, S.W. Davis, and B. Turrin, Cyclic aggradation and downcutting, fluvial response to volcanic activity, and calibration of soil-carbonate stages in the western Grand Canyon, Arizona, Quaternary Research, v. 53, 23–33, 2000.

Pederson, J., Karlstrom, K., Sharp, W., and McIntosh, W., 2002. Differential incision of the Grand Canyon related to Quaternary faulting – Constraints from U-series and Ar/Ar dating. Geology, v. 30, p. 739–742.

Stenner, H.D., Lund, W.R., Pearthree, P.A., and Everitt, B.L., 1999, Paleoseismologic investigations of the Hurricane fault in northwestern Arizona and southwestern Utah: Arizona Geological Survey Open-File Report 99-8, 137 p