HR: 12:05h
AN: T52A-08    [Abstracts]
TI: Deformation of Fluvial Terraces over Active Folds, Tian Shan - Tarim Foreland
AU: * Scharer, K
EM: kscharer@darkwing.uoregon.edu
AF: University of Oregon, Dept Geological Sciences 1272 U of O, Eugene, OR 97405 United States
AU: Burbank, D
EM: burbank@crustal.ucsb.edu
AF: UC Santa Barbara, Dept. Geological Sciences UCSB- Bldg 526, Santa Barbara, CA 93106 United States
AU: Jie, C
EM: chenjie@eq-igl.ac.cn
AF: Institute of Geology, China Earthquake Administration, State Key Lab of Earthquake Dynamics, Beijing, 100029 China
AU: Weldon, R
EM: ray@newberry.uoregon.edu
AF: University of Oregon, Dept Geological Sciences 1272 U of O, Eugene, OR 97405 United States
AB: Near the Chinese city of Kashgar, the southernmost expression of contraction between the Southern Tian Shan and the Tarim Basin is expressed as a series of four folds in Quaternary deposits. Exposed in transverse canyons, the folds reveal a generally coarsening upward suite of sediments, from gypsiferous mudstones in the core to fine grained sandstone dominating the limbs and a capped by the resistant Xiyu conglomerate. Through geologic mapping across seven transects we observed only one major unconformity. The unconformity is temporally variable across the region, occurring just below the Xiyu conglomerate in the west, but expressed in the youngest sediments above the conglomerate 150 km to the east. Magnetostratigraphic dating of the eastern area suggests that folding did not begin there until ~1.2 Ma. Since the growth strata are not sufficiently exposed to allow us to discern the kinematic style of anticlinal growth, we used fluvial terraces across three of the anticlines as kinematic markers for the recent evolution of the folds. We present geometric models of anticlinal growth and the resultant deformation of terraces successively emplaced on the growing folds. Detachment folding by limb rotation and limb lengthening, as well as combinations of these end members, produces very different geometric results. Both the architecture of the deformed terrace flights and the angular relationships between the pregrowth bedding inclination and the terraces are identifying characteristics of the style of fold growth. Application of these models to the Chinese folds and terraces suggests a combination of limb rotation and hinge migration is active. The fixed width of the folds along strike with a 10X range in shortening suggests material passes through fixed hinges at the synclinal axial surfaces. Progressively tilted terraces (up to 8 degrees) towards the core of the tight folds indicate limb rotation, possibly accommodated by hinge migration at the base of the steeply dipping fold limbs. Diapiric movement of gypsiferous mudstone through the core of the folds may facilitate the great vertical growth of the anticlines.
DE: 8005 Folds and folding
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting