HR: 12:05h
AN: T42B-08    [Abstracts]
TI: Intracontinental Strain Localization due to Heterogeneous Strength II: Evidence From Suture Reactivation in the Loqzung Range, Western Tibet
AU: * Raterman, N
EM: raterman@geology.ucdavis.edu
AF: University of California, Davis, Dept. of Geology One Shields Ave, Davis, CA 95616 United States
AU: Cowgill, E
EM: cowgill@geology.ucdavis.edu
AF: University of California, Davis, Dept. of Geology One Shields Ave, Davis, CA 95616 United States
AU: Ding, L
EM: dinglin@mail.igcas.ac.cn
AF: Chinese Academy of Sciences, Inst. of Geology and Geophysics, Beijing, 100029 China
AU: Zhang, H
EM: dinglin@mail.igcas.ac.cn
AF: Chinese Academy of Sciences, Inst. of Geology and Geophysics, Beijing, 100029 China
AB: The active, left-slip Altyn Tagh fault is a first-order structure within the Indo-Asian collision zone. Reconstruction of mid-Paleozoic and early Mesozoic arcs displaced by 475$\pm$70 km along this fault predicts that early Tertiary left-slip along the Altyn Tagh fault was absorbed by a S-directed thrust belt at the SW tip of the fault, producing the marked asymmetry in the N-S width of the Tibetan Plateau seen today. To test this prediction, we have conducted 1:100,000-scale structural mapping in the Loqzung range, which lies in western Tibet between the Karakorum range to the south and the western Kunlun Shan range to the north. Our mapping supports four conclusions about Mesozoic assembly and post-Cretaceous deformation of the NW-SE striking western Loqzung Range. First, the range coincides with a major tectonic boundary: the Taaxi-Qiaoertianshan-Hongshanhu (TQH) suture between the Qiangtang block to the south and the Songpan-Ganzi subduction-accretion complex to the north. In the Loqzung range, the TQH suture juxtaposes Jurassic m‚lange and late Paleozoic diamictites of Gondwanian affinity to the south against Carboniferous strata to the north that have previously been reported to show Cathaysian affinity. The location of this suture has formerly been disputed. Second, the range shows evidence of both S- and N-directed post-Cretaceous deformation, although the S-directed deformation predominates. Along the north edge of the range, a S-directed thrust places Carboniferous deposits over Cretaceous strata that are deformed by S-vergent folds. Post-Cretaceous, shortening has also reactivated the S-dipping TQH suture, thrusting Jurassic m‚lange that lies to the south of the Loqzung range northwards over folded Cretaceous rocks that lie within the range to the north. Ongoing work aims to establish the timing relations between the N- and S-directed phases of deformation. Third, the structural style of post-Cretaceous deformation within the Loqzung range varies along strike and appears to be controlled by underlying basement structure. Specifically, the structural style in the SE portion of the range is characterized by tightly folded Cretaceous deposits whereas to the NW these units are deformed by broad, upright folds. Although regional fold axes generally trend parallel to the TQH suture, in detail their orientations vary along strike, trending N70W in the SE and N50W in the NW. We estimate that post-Cretaceous shortening varies along strike from roughly 45 to 20 percent from the SE to the NW. Thrust-displacement along the reactivated TQH suture also appears to vary along strike, placing Jurassic m‚lange over younger Cretaceous units to the SE and older Cretaceous units to the NW. Fourth, the western Loqzung range is bounded at its southeast end by the E-W striking, active, left-slip Ghoza Co fault. This fault separates older, less deformed Cretaceous deposits to the north from younger, more intensively deformed Cretaceous units to the south. Our field observations suggest that the location, geometry, and structural style of post-Cretaceous deformation in western Tibet was controlled by the TQH suture. Heterogeneous crustal strength thus appears to have been important in influencing Cenozoic deformation in the region.
DE: 9320 Asia
DE: 8005 Folds and folding
DE: 8010 Fractures and faults
DE: 8102 Continental contractional orogenic belts
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting