The deep structure of the Earth's Mantle underneath the Rio Grande Rift, New Mexico.
The RISTRA 1 experiment was a 1999-2001 950 km-long NSF-funded deployment of 54 seismographs with approximately 18 km station spacing across the southwestern U.S., approximately between Lake Powell, UT and Pecos, TX. The seismographs, provided by the IRIS PASSCAL program, recorded seismic signals from naturally occurring earthquakes from around the world and used these seismograms to image the deep earth. The underlying grey-scale image shows the Earth's abrupt geologic discontinuities (obtained using a technique called "receiver functions"), and shows that the thickness of the crust is 45 to 50 km beneath both the Colorado Plateau and the Great Plains, but thins dramatically to a minimum of 35 km centered nearly directly beneath the Rio Grande rift (RGR) axis. Variations in arrival times of signals from distant earthquakes sweeping across the array also enabled RISTRA researchers to perform a tomographic (CAT-scan-like) image of fast and slow (to first order cold and hot) structures within the Earth to depths of many hundreds of kilometers (underlying color image). Tomographic imaging shows a broad low velocity region centered beneath the rift axis that extends from the Tularosa Basin to the Jemez Lineament. These observations suggest that the lower crust and mantle beneath the RGR has extensively deformed, in a symmetric manner, about the rift axis with the opening of the RGR during the past approximately 30 million years. The relatively flat upper mantle discontinuities at 410 km and 670 km would be expected to be deflected by any large temperature anomalies at those depths. This, along with the velocity tomography image, indicates that there is not a large-scale, very-deep-seated thermal anomaly beneath the RGR, and that the rift is instead primarily an uppermost (200 km or shallower) mantle feature. Recent results such as this suggest that complex mantle structures may be ubiquitous in the western U.S. upper mantle, and that uncovering their nature will be a prime target for EarthScope and related science efforts in the next decade. Images after Wilson et al., 2004 and Gao et al., 2004.
Gao, W., Grand, S., Baldridge, S., Wilson, D., West, M., Ni, J., Aster, R., An upper mantle seismic cross section through the Colorado Plateau, Rio Grande Rift, and Great Plains, J. Geophys. Res., 109 (B3), p.B03305, 2004.
Wilson, D., Aster, R., Ni, J., Grand, S., West, M., Gao, W., Baldridge, W.S., Semken, S., Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau, submitted to JGR, 2004.
Submitted by Rick Aster, Geophysics Program, New Mexico Tech |
