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Degrees from epicenter time in minutes UT53 UT52 UT51 AZ49 AZ48 AZ47 AZ46 NM43 NM42 NM40 NM39 NM38 NM37 NM33 NM32 NM31 NM30 NM27 NM26 NM24 NM23 NM22 NM20 NM17 NM15 NM10 NM08 TX06 TX05 TX03 TX01 2.5 km/s 3 km/s 4 km/s 5 km/s 8 km/s 35 36 37 38 39 40 41 42 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 Northwest Southeast Rio Grande Rift 112° 108° 104° 100°W 30° 32° 34° 36° 38° 40° Great Plains Colorado Plateau Basin and Range Rocky Mtns. 114° #54 #01 TX CO NM AZ UT Mexico Figure 1 Tectonic prov- inces of the southwestern U.S. The Great Plains mark the western edge of the stable North American craton. Cenozoic exten- sion is evident throughout the region, particularly in the Basin and Range. Despite this regional extension, the Colorado Plateau has remained relatively undeformed as it has rifted away from the Great Plains. The result is a concentrated extension in the Rio Grande Rift. The RISTRA project examined a 950 km transect across these regions using 57 broadband PASSCAL seismic stations. The strike of the array is in line with earthquakes in both directions from the Pacific Rim. Figure 2 Seismic record section from the Mw 6.0 event in Kodiak, Alaska on 2000/11/06. Rayleigh waveforms vary considerably across the RISTRA array suggesting complex structure. Phase velocities are calculated for a moving window of vertical component seismograms. Blue box indicates the bin of stations stacked in Figure 3. Figure 3 Phase velocity stack function for four events. For each event, a bin of vertical component wavefields are slant-stacked over a range of slownesses in the fourier domain similar to the approach of McMechan & Yedlin and Herrmann & Ammon. Contour interval is 0.1. Maximum stack value is 1. Events A and B stack coherently from 10 to ~100 seconds. C and D stack well for a limited range of periods. To make use of all information we sum the contour maps for 29 events before extracing phase velocities. This multiple event approach also provides error estimates. Background & Data 10 20 30 50 70 100 150 2.5 3 3.5 4 4.5 5 Period (s) 2000/11/06 - 11:40 2.5 3 3.5 4 4.5 5 2000/05/12 - 18:43 2.5 3 3.5 4 4.5 5 2001/04/09 - 09:00 2.5 3 3.5 4 4.5 5 Phase velocity (km/s) 1999/10/13 - 01:33 A B C D 10 20 30 50 70 100 150 Period (s) 10 20 30 50 70 100 150 Period (s) 10 20 30 50 70 100 150 Period (s) 1 New Mexico State Univ., Dept. of Phys., Las Cruces, NM 88003 2 Los Alamos National Laboratory, EES-1, Los Alamos, NM 87545 3 New Mexico Insitute of Mining And Tech., Socorro, NM 87801 4 Univ. of Texas, Austin, Dept. of Geo. Sciences, Austin, TX 78712 5 Lawrence Livermore Natl. Lab., Earth Sci. Div., Livermore, CA 94550 6 Dine College, Division of Natural Sciences, Shiprock, NM 87420 Contact: [email protected] S61A-1116 Structure of the Uppermost Mantle Beneath the Colorado Plateau, Rio Grande Rift and Great Plains Michael West 1 , James Ni 1 , Scott Baldridge 2 , Dave Wilson 3 , Wei Gao 4 , Steve Grand 4 , Rick Aster 3 , Rengin Gok 5 ,Steve Semken 6 , John Schlue 3 20 40 60 80 100 120 140 160 180 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 Period (s) Phase velocity (km/s) Colorado Plateau Rio Grande Rift Great Plains = 1 σ Figure 4 Interstation phase velocity in each province along the array. Curves show the mean velocity across a third of the array corresponding roughly to the Colorado Plateau, Rio Grande Rift and Great Plains. Bootstrap errors are estimated from 29 events. Variations of 0.3 km/s are observed between regions. The Rio Grande Rift is slower than the cratonic Great Plains at all periods. The Colorado Plateau is similar to the Great Plains at < 80 seconds. Long periods however show velocities more in common with the rft. Phase velocities Figure 5 Phase velocity as a function of distance along the array. For periods up to 40 s, velocities are determined from a moving bin of five stations. At periods greater than 40 s, nine stations are used. An increased apperture for long periods is required to minimize errors. The bin widths impose a maximum resolution of ~75 km for short periods and ~150 km for long periods. Red lines mark the three regions averaged in Figure 4. 0 0.05 0 50 100 150 200 250 300 350 0.05 0.05 0.05 Depth (km) Sensitivity 20 s 50 s 90 s 150 s 400 300 200 100 0 100 200 300 400 20 40 60 80 100 120 140 160 180 NM41 3.6 Phase velocity (km/s) NM27 UT54 4.2 UT53 4.0 UT52 NM26 UT51 TX01 AZ50 4.4 AZ49 NM25 AZ48 NM28 AZ47 NM07 AZ46 TX06 AZ45 TX05 NM44 TX04 NM43 TX03 NM42 NM29 3.8 NM15 NM40 NM14 NM39 NM13 NM38 NM12 NM37 NM11 NM36 NM10 NM35 NM09 NM34 NM08 NM33 TX02 NM32 NM24 NM31 NM16 NM30 NM23 NM22 NM21 NM20 NM19 NM18 NM17 Period (s) 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Distance along array (km) Northwest Southeast Figure 6 Sensitivity kernels showing the dependence of phase velocity on shear velocity structure. 20 s waves are most sensitive to the mid-crust while 150 s waves are sensitive to depths of 200-300 km. The layers reflect the model parameterization which has increasing layer thicknesss with depth. The phase velocity profile at each station (Figure 5) was inverted using a damped least- squares approach and kernels similar to this example. Crustal thickness and depth of sedimentary basins were defined a priori using the receiver function results of Wilson et al. (see poster S61A-1113). Figure 7 Shear velocity structure of the crust and uppermost mantle. Velocity structure obtained by inverting phase velocities in Figure 5. In the crust, velocities are generally less near the rift where the crust is thinnest. The lower crust beneath the Great Plains is very fast (> 4.0 km/s) in agreement with prior studies. The mantle beneath the Great Plains is also fast reflecting its cratonic history. No clear asthenospheric channel is present. Beneath the Colorado Plateau, the top of the mantle has velocities of 4.55 km in agreement with Pn velocities of 8.1 km from this and previous studies. However at depths of 150-250 km, the Colorado Plateau is underlain by material that is almost 10% slower than at comparable depths beneath the Great Plains. The lowest mantle velocities (~4.2 km/s) are found beneath the rift just under the Moho. Low velocities beneath the rift do not extend to depth. 400 300 200 100 0 100 200 300 400 0 100 200 300 400 Distance (km) Depth (km) 4.2 4.4 4.6 4.8 4.2 <3 shear velocity in crust (km/s) shear velocity in mantle (km/s) Mantle structure Northwest Southeast 100 200 300 0 A E D C B crust lithospheric mantle asthenosphere Colorado Plateau Rio Grande Rift Great Plains Figure 8 Intepreted section. A: High velocities of 4.6-4.7 km/s beneath the Great Plains indicate a thick shield-like lithosphere of at least 200 km. B: Great Plains asthenosphere velocities are faster than anywhere else along the array indicating a sharp edge to the tectonically stable region of North America. C: The lithosphere beneath the Colorado Plateau is ~140 km thick. The fast velocities indicate cold lithosphere relative to the adjacent Basin and Range province and Rio Grande Rift. The inherited strength of the lithosphere has allowed the Colorado Plateau to remain undeformed despite regional extension. D: Under the Colorado Plateau below 150 km, a low velocity zone (4.2 km/s) indicates warm asthenosphere. We propose that low density associated with this mantle anomaly provide a buoyant force which partially supports the plateau's present high elevation. E: Beneath the rift, the thermal lithosphere shallows to within about 20 km of the base of the crust (thinned as well). Sub-Moho velocities of 4.2 km/s are consistent with Pn observations and indicate temperatures elevated by a few hundred degrees and/or small amounts of partial melt. F: Asthenospheric velocities beneath the rift do not connect to a broad deep plume or sheet-like upwelling. However small convective cells could exist at scales smaller than the resolution of the surface waves. F low velocity zone
