HR: 14:10h
AN: T22C-03    [PDF]
TI: Detrital zircon provenance analysis of Oligocene sandstone in the eastern Xorkol basin and its implications for the magnitude of displacement along the eastern Altyn Tagh fault
AU: * Yue, Y
EM: yongjun@pangea.stanford.edu
AF: Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Stanford, 94305-2115
AU: Graham, S A
EM: graham@pangea.stanford.edu
AF: Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Stanford, 94305-2115
AU: Ritts, B D
EM: ritts@cc.usu.edu
AF: Department of Geology, Utah State University, 4505 Old Main Hill, Logan, 84322-4505
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, 94025
AB: Oligocene strata in the eastern Xorkol basin north of the Altyn Tagh fault (ATF) consist of interbedded pebble to cobble conglomerate and red mudstone deposited in a braided fluvial environment. Ubiquitous imbrication in the conglomerate beds yields paleocurrent measurements indicative of north to northwest-directed paleoflow, suggesting sediment derivation from the southern side of the ATF. Clast types include slate, phyllite, limestone, dolomite, metavolcanic rocks and metasandstone, indicating a low-grade metamorphic source terrane with few granitic intrusions. In order to better characterize the source of Oligocene detritus, a sandstone sample was collected in the uppermost Oligocene strata for SHRIMP detrital zircon age dating. Most of the zircon grains are zoned in structure and pink in color with few fractures. Thirty spot analyses yield a characteristic age distribution for the sandstone. Except for 5 analyses which give discordant ages between 650 Ma and 830 Ma, 19 analyses are clustered around 917 Ma with a standard deviation of 42 Ma, and the other 6 analyses lie between 1100 Ma and 1700 Ma. The zircon grains have a mean Th/U ratio of 0.42 with standard deviation of 0.19, typical for zircons of magmatic origin. The resulting zircon age distribution puts important constraints on the source of the Oligocene detritus in the eastern Xorkol basin. The lack of Paleozoic zircon ages eliminates the main part of the central Qilian Shan, the southern Qilian Shan and Qaidam basin as possible sources because they contain widespead Paleozoic plutons. It also eliminates the Ordovician and Silurian volcaniclastic rocks of the northern Qilian Shan as a possible source inasmuch as their detrital zircons, shed from a synchronous magmatic arc in the central Qilian Shan, are predominantly early Paleozoic in age. Therefore, the only possible sources are Cambrian and Neoproterozoic strata in the southern part of the northern Qilian Shan and the northernmost part of the central Qilian Shan, where the Neoproterozoic strata contain zircon grains whose U/Pb ages are concordant at ~930 Ma and zircon grains whose ages range between 1000 and 1600 Ma. An Oligocene piercing point is thus deduced by realigning this possible source and the Xorkol basin, consistent with 350-400 km of left-lateral offset along the eastern segment of the ATF. Determination of the magnitude of displacement along the eastern ATF is critical for understanding the nature of this fault and its role in the formation of the Tibet Plateau. Because the areally small northern part of the northern Qilian Shan beyond our piercing point is unlikely to accommodate a significant portion of the 350-400 km of left-lateral offset along the ATF, northeastward extrusion must have existed to assist in accommodating this offset, indicating that the ATF was an extrusion boundary, at least in the early Neogene. On the other hand, the 350-400 km of left-lateral offset is smaller than the offset along the western segment of the ATF. This decrease in offset indicates that the ATF is also a boundary across which slip was transferred into crustal shortening of the Qilian Shan and the Qaidam basin.
DE: 8110 Continental tectonics--general (0905)
DE: 8115 Core processes (1507)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting