HR: 14:25h
AN: G13C-04    [Abstracts]
TI: Tenfold Disparity Between Decadal InSAR and Millennial Morphochronologic Slip-Rates on the Karakorum Fault
AU: * Chevalier, M
EM: chevalie@ipgp.jussieu.fr
AF: IPGP, laboratoire de tectonique case 89 4 place jussieu, paris cedex 05, 75252 France
AU: Ryerson, F J
EM: ryerson1@llnl.gov
AF: IGPP-LLNL, East avenue 7000, livermore, CA 94550 United States
AU: Tapponnier, P
EM: tappon@ipgp.jussieu.fr
AF: IPGP, laboratoire de tectonique case 89 4 place jussieu, paris cedex 05, 75252 France
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: IGPP-LLNL, East avenue 7000, livermore, CA 94550 United States
AU: Van Der Woerd, J
EM: Jerome.Vanderwoerd@eost.u-strasbg.fr
AF: IPGS, Tectonique Active 5, rue Rene Descartes, strasbourg, 67084 France
AU: Li, H
EM: haibing@ipgp.jussieu.fr
AF: Ministry of land and resources, ?, beijing, ? China
AU: Liu, Q
AF: Total exploration, ?, beijing, ? China
AB: The role of the Karakorum Fault in the kinematics of present-day deformation in Tibet is debated. Although it is the main dextral strike-slip fault north of the Himalayas, recent InSAR data are interpreted to suggest that it is barely active, moving at a rate of 1 $\pm$ 3 mm/yr. Surface exposure dating of moraines and terraces south of Bangong lake tell a different story, suggesting it slips ten times as fast. At one site on the west side of the Gar graben (32$\deg$3' N-80$\deg$1' E, 4365 m-4760 m), at the foot of the Aliyari Range, we sampled quartz-rich, rooted blocks on the lateral moraine crests of a large till complex. The moraines are offset right-laterally relative to the valley of the Manikala Daer Glacier. Twenty-seven samples were dated using cosmogenic 10Be. The offsets of 2 well-defined moraine crests (M1 and M2E), obtained from retro-deformation of 1 m-resolution IKONOS images, are 220 $\pm$ 10 m and 1520 $\pm$ 50 m, respectively. The 10Be exposure ages fall in distinct clusters: 21 $\pm$ 0.1 ka and 40 $\pm$ 3 ka on M1, and 140 $\pm$ 5.5 ka and 180 $\pm$ 14 ka on M2. The ages of maximum sample abundance correlate remarkably well with the ages of the coldest periods derived from proxy paleo-temperature records (SPECMAP), suggesting that the Manikala glacier crossed the fault to emplace moraines northeast of the range front only during the LGM and penultimate glacial maximum (late stage 6), and implying little erosion. Pairing the abandonment ages of the two moraines ($\sim$ 21 and 140 ka) with their offsets yields concordant dextral slip-rates of 10.9 $\pm$ 0.6 and 10.5 $\pm$ 0.5 mm/yr. The resulting, average slip rate (10.7 $\pm$ 0.7 mm/yr) we derive for the last 150 ka is comparable to the geological rate obtained by Lacassin et al., [2004] (10 $\pm$ 3 mm/yr in the last 34 Ma) and to the GPS geodetic rate determined by Banerjee and Burgmann, [2002] (11 $\pm$ 4 mm/yr). It is almost 3 times greater than that measured in India by Brown et al., [2002] (4 $\pm$ 1 mm/yr) and three times smaller than that predicted by Liu Qing [1993] (30 mm/yr). The $\sim$ 11 mm/yr slip-rate along the Ayilari range should be considered a minimum because the site is situated on one side of the Gar graben, and at least two branches of the Karakorum fault system splay off into that graben north of Gar. Similar morphochronologic evidence at other sites north and south of Gar yields results in keeping with a millennial slip-rate of at least 1 cm/yr on the fault, implying that extrusion of Tibet between the converging Indian and Tarim plates at a comparable rate is not in doubt. That the GPS and InSAR values differ by a factor of ten suggests that strong monsoonal tropospheric effects might bias the InSAR data. If the tenfold disparity between decadal and millennial rates is real, then it might reflect "go/stop/go" slip-rate fluctuations, over periods comparable to or longer than the seismic cycle, related to the mechanical behavior of the fault zone integrated between the base of the crust and the surface.
DE: 7221 Paleoseismology
DE: 8107 Continental neotectonics
DE: 8159 Rheology--crust and lithosphere
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting