NEW_Accomplishments.indd - IRIS

SURFACE OF THE EARTH: NORTH AMERICA
2006 IRIS 5-YEAR PROPOSAL
Modern Basalt Extraction Structures in the Southern Rocky Mountains:
Multi-band Images from the Jemez Lineament
Alan Levander, Colin A. Zelt, Maria B. Magnani • Rice University
Ken Dueker, Huaiyu Yuan • University of Wyoming
The CD-ROM seismic experiments targeted Paleoproterozoic suture zones in the western U.S. in a north-south study corridor
from central New Mexico to central Wyoming. Seismic reflection, refraction, and teleseismic measurements were made
across the Jemez Lineament in northern New Mexico, a linear trend of modern volcanics extending SW from southern Colorado
to Arizona. The Jemez Lineament spatially coincides with the southern edge of the suture between the Yavapai and Mazatzal island
arc terranes thought to be one of the Paleoproterozoic
assembly boundaries remaining from initial continental accretion.
Karlstrom and Humphreys (1998) have speculated
that the assembly boundaries have profoundly influenced
Cenozoic tectonism in the western U.S., noting the correlation
of NE-SW upper mantle tomography anomalies with
geochemical boundaries and mapped suture zones in the
Southern Rocky Mountains.
The reflection data image a Paleoproterozoic bivergent
orogen occupying more than half the crust marking the
Yavapai-Mazatzal orogeny, and bright upper crustal reflections
that we interpret as Quaternary basaltic sills (Magnani
et al., 2004). Refraction velocities in the upper mantle
under a slightly thinned crust suggest that the upper mantle
contains 1% partial melt (Hammond and Humphreys, 2000;
Levander et al., 2005). In the same upper mantle region the
P and S teleseismic tomography models show large-magnitude
low-velocity anomalies (Yuan and Dueker, 2005). A
pre-stack depth-migrated receiver function image shows a
series of subhorizontal, very bright, negative-polarity upper
mantle conversions extending from the Moho to depths of
~125 km, roughly corresponding to the tomography low
velocity region (Levander et al., 2005). We interpret this
complex series of converters in the upper mantle as the
source zone for the Quaternary basaltic magmas found at
the Jemez Lineament. We speculate that the paleo-suture
zone left from continental accretion acts as a crustal conduit
for basaltic magmas to pass from the mantle into the crust,
form sills, and erupt.
Levander, A., C. A. Zelt, and M.B. Magnani, Crust and upper mantle velocity structure of the Southern Rocky Mountains from the Jemez Lineament to the
Cheyenne Belt. Lithospheric Structure and Evolution of the Rocky Mountain Region. K. E. Karlstrom and G. R. Keller. Washington, D.C., American
Geophysical Union, 2005.
Levander, A., F. Niu, C.-T. A. Lee, and X. Cheng, Imag(in)ing the Continental Lithosphere, Physics of the Earth and Planetary Interiors, accepted, 2005.
Magnani, M.B., K.M., Miller, A. Levander, and K. Karlstrom, The Yavapai-Mazatzal boundary: A long-lived tectonic element in the lithosphere of southwestern
North America, Geol. Soc. Am. Bull., 116, 1137-1142, 2004.
Yuan, H., and K. Dueker, Upper mantle tomographic Vp and Vs images of the middle Rocky Mountains in Wyoming, Colorado, and New Mexico: Evidence for
a thick heterogeneous chemical lithosphere, in Lithospheric Structure and Evolution of the Rocky Mountain Region, edited by K. E. Karlstrom and G. R.
Keller, Washington, D.C., American Geophysical Union, 2005.
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