HR: 16:00h
AN: GP44A-01    [Abstracts]
TI: Apatite Fission-Track Thermochronology of the Southern Verkhoyansk Fold-and-Thrust Belt, Russia
AU: * Toro, J
EM: jtoro@wvu.edu
AF: Geology and Geography, West Virginia Univ., Morgantown, WV 26506 United States
AU: Prokopiev, A
EM: prokopiev@diamond.ysn.ru
AF: Diamond and Precious Metal Geology Institute, 39 Lenin Av., Yakustk, 678980 Russian Federation
AU: Colgan, J
EM: jcolgan@pangea.stanford.edu
AF: Geol. and Env. Sciences, Stanford University, Palo Alto, CA 94305 United States
AU: Dumitru, T
EM: trevor@pangea.stanford.edu
AF: Geol. and Env. Sciences, Stanford University, Palo Alto, CA 94305 United States
AU: Miller, E L
EM: miller@pangea.stanford.edu
AF: Geol. and Env. Sciences, Stanford University, Palo Alto, CA 94305 United States
AB: The Verkhoyansk fold-and-thrust belt of eastern Yakutia is one of the largest Mesozoic compressional belts. It involves an immense prism of strata deposited on the passive margin of the North Asian craton from Late Proterozoic through Jurassic time. Current models suggest that shortening in the Verkhoyansk was the result of collision and accretion of terranes against the Asian margin. The onset of deformation is constrained by the beginning of rapid subsidence in the Pri-Verkhoyansk foreland basin during Late Jurassic and by a poor quality $151\pm1$ Ma $^{40}$Ar/$^{39}$Ar age from the Sette-Daban zone of the Southern Verkhoyansk. The central part of the fold-and-thrust belt is underlain by low-grade Late Paleozoic basinal clastic rocks intruded by granodioritic plutons (119-123 Ma, $^{40}$Ar/$^{39}$Ar biotite). To the west there is a 25km-wide belt of intensely deformed Early to Middle Paleozoic basinal strata known as the Sette-Daban zone, which has a fan-shaped cleavage pattern. Still further west, lies the 40-80 km wide Kyllakh Zone composed of a series of west-vergent thrusts that involve Late Proterozoic-Jurassic platformal strata. Conodont alteration indices reveal that there were up to 14 km of tectonic and sedimentary burial in the Sette-Daban, while vitrinite reflectance data show that foreland basin sedimentation was minimal. We carried out apatite fission-track (AFT) thermochronology on 17 samples collected along a 120km long transect between the Allakh-Yun' and Yudama rivers. One apatite sample from Lower Jurassic sandstone of the foreland is not reset, yielding an age of $210\pm10$ Ma. Five samples from the frontal thrust sheets yielded central ages ranging from $78\pm6$ to $91\pm9$ Ma. However, all these samples have mean track-length distributions of less than 13.2 micrometers, indicative of relatively long residence within the PAZ. These ages reflect erosional denudation as a response to thrust uplift beginning in mid- to Late Cretaceous time followed by slow cooling during the Tertiary. A high-quality AFT age from the Tarbagganakh pluton is $90.5\pm3.2$ Ma. Track-length modeling shows that this sample underwent rapid cooling from 120 Ma (approximate intrusive age) to about 80 Ma, followed by slow cooling after the Late Cretaceous. Five other ages from the Allakh-Yun' zone are younger, ranging from $74\pm3$ Ma to $49\pm3$ Ma. The older ages are compatible with those from the frontal thrusts. Track-length modeling of the youngest age reveals two episodes of rapid cooling, one in the Early Cretaceous and a second one between 50 and 30 Ma. This second cooling episode may reflect late-stage deformation. Four samples from Sette Daban zone also yielded younger AFT central ages ranging from $45\pm6$ to $60\pm6$ Ma. Our preliminary interpretation is that these latest Cretaceous-early Tertiary ages reflect uplift associated with deformation along the N-S striking Burkhala left-lateral fault which cuts this portion of the thrust-belt.
DE: 9320 Asia
DE: 8102 Continental contractional orogenic belts
DE: 8110 Continental tectonics--general (0905)
DE: 1035 Geochronology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting