Craig H. Jones
Ph.D. Massachusetts Institute of Technology, 1987
Associate Professor, Geological Sciences
E-mail: cjones@colorado.edu
Office: ESCI (Benson) 440C
Phone: 303-492-6994
Web: Prof. Jones
Research Interests
Jones’s research focus on the deformation of continents with a special interest in the western United States.
Current Research: Origins of the Sierra Nevada
In the six to seven years since the last portable seismometer was removed from the Sierra, seismological analysis has revealed that the entire eastern side of the range (the high part) has a relatively thin crust over a fairly low wavespeed upper mantle. The granite that makes up so much of the “Range of Light” was extracted from lower crustal melts that should have left a residual thick mass of dense, quartz-poor rock, but the seismological work confirms that this material is missing over the entire length of the range. Because volcanic rocks sampled the residuum some 8 million to 12 million years ago, its absence today suggests that it has been removed recently.
The seismic images can be used more quantitatively. CIRES Ph.D. graduate student Will Levandowski and I used the seismic wavespeed variations to calculate the contribution to the elevation of the range from within the crust and from the mantle, finding that each had contributed roughly half to the modern elevation of the range. Because the crustal part had been made about 80 million years ago, this suggests that roughly half of the modern elevation of the Sierra arose as residuum that used to underlay the mountains sank.
But where did that residuum go, and how was it removed? Again using the seismic image, I, former master’s student Heidi Reeg, and collaborators determined the equivalent of the mass anomaly removed from under the range and compared it to the anomaly present within a seismic anomaly in the upper mantle under the southwestern part of the range and the adjacent San Joaquin Valley. This analysis was unusual in taking care to avoid errors from the kinds of ambiguity in this kind of imaging that are evident in the section shown here. The mass anomaly removed and that present in the upper mantle today are the same, within uncertainties.
A surprising finding came as I helped co-principal investigator Jeff Unruh (Lettis Consultants International) and Egill Hauksson (Caltech) understand the seismicity near this mantle anomaly. Instead of variations in fault orientations consistent with the crust being pulled down, the seismicity suggests that the area might be moving upwards. If this is so, it means that less-dense crust has been pulled into the mantle anomaly and only now is starting to rebound as the mantle anomaly descends farther into the earth. Such a reversal has been suggested by theoretical work but has not been observed prior to this.
Then University of Arizona graduate student Andy Frassetto writes field notes as SNEP backcountry station 65 is removed from near Nelson Lake in Yosemite National Park in late summer 2007. Photo credit: Craig Jones/CIRES
Publications
Click here for a complete list of published works »
Dr. Jones is a professor at the University of Colorado at Boulder.
