HR: 17:05h
AN: T14C-05    [Abstracts]
TI: Along-Strike Variation in Geometry and Kinematics of a Major, Active Intracontinental Thrust System: the Pred-Terskey Fault Zone, Kyrgyz Tien Shan, Central Asia
AU: * Burgette, R J
EM: rburgett@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Weldon, R J
EM: ray@newberry.uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Abdrakhmatov, K Y
EM: kanab@elcat.kg
AF: Kyrgyz Institute of Seismology, National Academy of Sciences, 720060, Asanbay 52/1, Bishkek, 720060 Kyrgyzstan
AU: Ormukov, C
EM: orumkov@netmail.kg
AF: Kyrgyz Institute of Seismology, National Academy of Sciences, 720060, Asanbay 52/1, Bishkek, 720060 Kyrgyzstan
AB: The Pred-Terskey fault zone defines the southern margin of the Issyk-Kul basin, extending eastward over 250 km from at least the Chu River to the Kazakhstan border, and appears to be one of the most active zones in the Kyrgyz Tien Shan. Despite a diversity of structural styles and changes of vergence at the surface, the lateral continuity and overall geometry of the zone is consistent with a single north vergent thrust at depth, which uplifts the Terskey Range and generally tilts the south margin of the basin to the north. This northward tilting of the margin is probably due to a flattening of the fault as it approaches the surface. In spite of historical quiescence, it is likely capable of producing great earthquakes. We have conducted detailed field mapping coupled with terrace profiling and dating at seven representative, well-exposed areas of the fault zone. Based on these field observations and satellite image and air photo interpretation along the entire zone, we identify three major divisions in structural style expressed at the surface. The western segment is typified by the Tura-Su, Ak-Terek and Ton areas. A series of left-stepping, south-vergent, basement-involved reverse faults and folds are uplifting the southern margin of the Issyk-Kul basin in this area. The resulting uphill-facing scarps have trapped and diverted many of the rivers flowing north from the Terskey Range. Tertiary strata and Quaternary geomorphic surfaces show consistent, progressive northward tilting across the entire zone. The west-central segment is represented by the Kajy-Say area. South-vergent reverse faults and a north-vergent backthrust have uplifted an arcuate granite block. Offshore of this area, the lake floor descends to a sharp break in slope with a low relief area at a depth of about 650 m. Late Quaternary geomorphic features do not show evidence of tilting. In contrast to the areas east and west, the major north-dipping thrust is likely planar over this segment and daylights at the lake floor break in slope. The east-central segment is exemplified by the Barskaun and Jety Oguz areas. A high angle reverse fault juxtaposes Paleozoic rock against Tertiary sediments. To the north, a thrust fault with a sinuous trace places north-dipping Tertiary rock over the nearly horizontal basin floor. Quaternary terraces in the hanging wall of this fault record progressive northward tilting. North of the thrust fault a series of anticlines are growing out of the basin sediments. The eastern segment, which includes the Jergalan River valley, lacks a low angle thrust fault at the basin margin. Along this segment, the basement reverse fault uplifts Paleozoic rock against Quaternary basin sediment. To the north of this range-bounding structure, late Quaternary terraces are offset by south-vergent scarps. We are calculating geologic slip rates for each of the seven sites along the Pred-Terskey zone by dating terraces and constructing structural models consistent with both the rock and terrace records. Based on preliminary radiocarbon dates, a prominent Jety Oguz River terrace is 50 +/- 10 ka. The terrace is tilted $0.5\deg$ relative to the modern river, and with the low angle fault branching off of the basement reverse fault at dips ranging between $45\deg$ and $90\deg$, the slip rate of this fault is 6 +/- 4 mm/yr. This is consistent with the GPS shortening rate across the Pred-Terskey zone at this longitude.
DE: 8005 Folds and folding
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting