HR: 15:10h
AN: G13C-07    [Abstracts]
TI: Kinematics of the southern Alaska constrained by westward-decreasing post-glacial slip-rates on the Denali fault, Alaska.
AU: * Meriaux, A
EM: meriaux1@llnl.gov
AF: Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, P.O. Box 808, L-206, Livermore, CA 94550 United States
AU: Sieh, K
EM: sieh@gps.caltech.edu
AF: Division of Geologic and Planetary Science, California Institute of Technology, MC 170-25 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: Rubin, C M
EM: charlier@geology.cwu.edu
AF: Department of Geological Sciences, Central Washington University, 400 University Way, Ellensburg, WA 98926 United States
AU: Ryerson, F J
EM: ryerson1@llnl.gov
AF: Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, P.O. Box 808, L-206, Livermore, CA 94550 United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, P.O. Box 808, L-206, Livermore, CA 94550 United States
AU: Meltzner, A
EM: meltzner@gps.caltech.edu
AF: Division of Geologic and Planetary Science, California Institute of Technology, MC 170-25 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: Taylor, M
EM: mtaylor@gps.caltech.edu
AF: Division of Geologic and Planetary Science, California Institute of Technology, MC 170-25 1200 E. California Blvd, Pasadena, CA 91125 United States
AB: Cosmogenic exposure age analyses of offset glacial moraines show that the Holocene slip-rate of the arcuate right-lateral Denali fault decreases westward from about 13 mm/yr at $144^{\circ}$W to about 7 mm/yr at $149^{\circ}$W. This pattern is consistent with a tectonic model in which the southern part of Alaska (the Wrangell Block) is translating northward relative to northern Alaska (North American plate) without rotation. It also implies that convergence rates across the Alaska Range's northern frontal thrust fault system increase significantly from east to west from east to west from about 5 to 14 mm/yr. Slip distribution along the Denali fault during the great 2002 earthquake was decidedly asymmetric. This distribution of dextral slip along the 220-km rupture - decreasing from about 10 m in the east to about 3 m in the west - suggested that the long-term slip rate on the Denali fault might also decrease westward. If true, this would have major implications for the kinematics of southern Alaska, in general, and the Alaska range, in particular. If the Wrangell block is rotating counter-clockwise, the slip-rate on the arcuate Denali fault would be roughly uniform along strike. If however, southern Alaska is translating northwestward without rotation, the slip-rate magnitude should diminish westward. To constrain these models, we sampled offset moraines for $^{10}{Be}$ cosmogenic exposure dating at two sites along the Denali fault. At Slate Creek (\approx$144^{\circ}$W), we sampled boulders, both upstream and downstream from the fault, on a moraine offset 156$\pm$20 m. The minimum exposure ages range from 7.8$\pm$0.5 to 12.3$\pm$0.8 ka and average 10.7$\pm$1.3 ka. About 235 km to the west, at Bull Creek (\approx$149^{\circ}$W), we collected large cobbles downstream from the fault, on a moraine offset 82$\pm$5 m. There, the minimum exposure ages range from 9.8$\pm$0.8 to 12.3$\pm$0.9 ka and average 11.0$\pm$0.8 ka. We corrected these minimum exposure ages for snow shielding, using historical snow cover data averaged over 30 years near our sites. Furthermore, to estimate the variation of snow cover over time, we included the changes in effective wetness over the last 11 ka, calculated from lake-level records and $\delta^{18}O$ variations from lakes north of the Alaska range. The time-averaged snow corrections increase the ages of the moraines by 9$\pm$0.3% at Slate creek and 12$\pm$0.3% at Bull creek. Division of the corrected ages into the offsets yields slip-rates of 13.2$\pm$4 mm/yr at Slate creek and 6.6$\pm$1.7 mm/yr at Bull Creek. Thus it is clear that the long-term rate of slip on the Denali fault decreases appreciably from east to west. These values are consistent with a kinematic model in which southern Alaska is translating northwestward without rotation at a rate of about 15 mm/yr. The rates also suggest progressive slip partitioning between the Denali fault and the active fold and thrust belt at the northern front of the Alaska range, with rates of convergence increasing westward from ~5 to 14 mm/yr between ~149 and $144^{\circ}W.
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 1035 Geochronology
DE: 1208 Crustal movements--intraplate (8110)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting