Cooperative Institute for Research in Environmental Sciences

Abstracts: 17

Structure of Western North America

Craig H. Jones, Anne F. Sheehan, G. Lang Farmer, Peter H. Molnar, Roger Bilham

Here in Colorado, where the mantle supports some of the highest topography in the U.S., the mantle may also be actively deforming the Earth's surface. Sheehan, Bilham and postdoc Fred Blume have been investigating whether strain is accumulating across the state. They completed an initial reoccupation of old geodetic markers with GPS equipment this past year that should soon allow us to know to what degree Colorado is accumulating strain that could produce earthquakes.

In the Sierra Nevada of California, geochemical work by Farmer and coworkers has shown that a puzzling suite of volcanic rocks that were erupted 3.5 million years ago (Ma) sampled an otherwise unmelted part of the mantle isolated from convective processes since the Precambrian (c. 1000 Ma). The unique petrological and geochemical characteristics of these rocks can be explained if the volcanics were generated when a thick layer of eclogite was removed from the base of the crust.

Jones and Farmer and a colleague have reinterpreted the tectonic history of much of California in light of this 3.5 Ma event. Removing the ~50 km thick layer of eclogite caused uplift of the Sierra at about that time. This in turn initiated extensional faulting in the area adjoining the Sierra on the east. The increased extension must be compensated by increased shortening elsewhere, which is consistent with the creation of much of the Coast Ranges about 3 Ma. Finally, changes in the lateral extent of the rigid Sierra Nevada-Great Valley block predict a decrease in slip rate on the San Andreas Fault and an increase in strike-slip east of the Sierra, a prediction compatible with available observations.

Molnar and Jones have further exploited the timing from the Sierran system to provide the first ever observational control on the rate of removal of mantle lithosphere. The 7 Myr duration of the removal of the eclogite, combined with its appearance in two anomalies extending to about 300 km depth, places constraints on the viscosity of the uppermost mantle. When combined with heat flow constraints, experimentally derived flow laws are probably consistent with the rapid deformation inferred.

Flow in the uppermost mantle can reveal connections between the crust and deeper mantle. Gradients in flow are found from measurements of seismic anisotropy made and compiled by Sheehan and a coworker. These reveal a peculiar pattern suggestive of major mantle upwelling under eastern Nevada under southwest-moving lithosphere.

At deeper levels, changes in the mineralogical structure of mantle materials with pressure produce discontinuities in seismological properties that will vary in depth depending on temperature. Sheehan, Molnar, and postdoc Hersh Gilbert have combined seismograms from across the southwestern U.S. to produce an image of the variations in the discontinuities at 410 and 660 km depth. In general, the depths of these discontinuities do not vary in a manner indicative of vertically coherent thermal anomalies. This suggests that the mantle in this region is not involved in whole-mantle convective flow. Continued work will address whether this is a result of layered convection or reflects some transience in this system as the plate boundary has changed from subduction to strike-slip motion.