Erchie Wang

Mesozoic sedimentary evolution of the northwest Sichuan basin: Implication for continued clockwise rotation of the South China block

We present new sedimentary data integrated into a regional Mesozoic stratigraphic framework to provide a detailed picture of spatio-temporal variations in deposition and depocenter migration of the northwest Sichuan basin. The Mesozoic sedimentary evolution is utilized to interpret basin subsidence history and to unravel coeval basin-margin tectonics. The northwest Sichuan basin, together with the Songpan-Ganzi terrane, behaved as a passive margin south of the Qinling Paleo-Tethys from late Paleozoic to early Middle Triassic times and then evolved into a peripheral foreland basin in response to collision of the North and South China blocks since the late Middle Triassic. Coeval with strong north-south contraction of the Songpan-Ganzi terrane in the Late Triassic, sinistral transpressional deformation of the Longmen Shan belt led to flexural subsidence of the adjacent western Sichuan basin. Renewed basin-margin fold-thrust activity triggered recurrence of flexural subsidence of the northwest Sichuan basin since the Middle Jurassic, with the depocenter eventually shifting to the northwestern corner of the basin in the Early Cretaceous. Sedimentary evolution of the northwest Sichuan basin and the basin-margin deformation imply that the South China block had been rotating clockwise relative to the North China block throughout the Mesozoic with an interim period of Early Jurassic tectonic quiescence. A model is advanced that invokes clockwise rotation of the South China block as a driver for tectonic evolution of both the basin and adjoining structural belts and provides an explanation for several salient features that are otherwise puzzling.

Late Cenozoic to Holocene deformation in southwestern Sichuan and adjacent Yunnan, China, and its role in formation of the southeastern part of the Tibetan Plateau

From at least 2–4 Ma to present, crust in the southeastern part of the Tibetan Plateau west of the convex-east Xianshuihe-Xiaojiang fault system has deformed internally and rotated clockwise around the eastern Himalayan syntaxis. The northwest-striking Ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78–100 km, of which ∼60 km is transferred to the Xianshuihe fault zone across a diffuse transfer zone, and ∼22–40 km is absorbed by bending of older structures and crustal shortening. Crustal shortening is expressed along and east of the eastern end of the Ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the Tibetan Plateau. A similar transfer of left slip into shortening occurs farther south across the Xianshuihe fault in the high mountains around and east of Gongga Shan (7556 m). The northwest-striking, convex-east, left-lateral Litang fault zone lies southwest of the Ganzi-Xianshuihe-Xiaojiang fault zone and appears to be less well developed but otherwise similar to the Ganzi fault zone.nnThe Batang, Chenzhi, and other northeast-striking right-lateral faults of small displacement occur within the rotating crustal fragment. Together with the left-slip faults, they accommodate east-west shortening northeast of the eastern Himalayan syntaxis. South of this region of shortening, the crust is extending to form grabens within the Dali and southern Xiaojiang fault systems and in the Tengchong volcanic province. The progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern Himalayan syntaxis.nnThe assemblage of structures in southwestern Sichuan geometrically resembles structures of Eocene to Miocene age in southern Yunnan that were positioned northeast of the eastern Himalayan syntaxis, similar to present-day southwestern Sichuan, at the time of their development. The similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past ∼50 m.y. Structures in Sichuan are less fully developed than older structures in southwestern Yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis. Deformation in these areas indicates that plateau formation has been complex, inhomogeneous, and diachronous at scales from 1000 km to less than 100 km.

Crustal structure across Longmenshan fault belt from passive source seismic profiling

[1] We analyse receiver functions from 29 broad-band seismographs along a 380-km profile across the Longmenshan (LMS) fault belt to determine crustal structure beneath the east Tibetan margin and Sichuan basin. The Moho deepens from about 50 km under Songpan-Ganzi in east Tibet to about 60 km beneath the LMS and then shallows to about 35 km under the western Sichuan basin. The average crustal Vp/Vs ratios vary in the range 1.75-1.88 under Songpan-Ganzi in east Tibet, 1.8-2.0 under the LMS, and decrease systematically across the NW part of the Sichuan basin to less than 1.70. A negative phase arrival above the Moho under Songpan-Ganzi and Sichuan basin is interpreted as a PS conversion from the top of a low-velocity layer in the lower crust. The very high crustal Vp/Vs ratio and negative polarity PS conversion at the top of lower crust in east Tibet are inferred to be seismic signatures of a low-viscosity channel in the eastern margin of the Tibetan plateau. The lateral variation of Moho topography, crustal Vp/Vs ratio and negative polarity PS conversion at the top of the lower crust along the profile seem consistent with a model of lower crust flow or tectonic escape.

Mesozoic large-scale lateral extrusion, rotation, and uplift of the Tongbai-Dabie Shan belt in east China

The tectonic convergence between the South China block and Qinling belt that occurred in Mesozoic time was not homogeneous along strike; it was mainly concentrated in the Shengnongjia and Hannan domes, which formed a pair of indentors, along which the South China block penetrated the Qinling belt. As a consequence, the Tongbai‐Dabie Shan belt, the eastern part of the Qinling orogenic belt, may have undergone large-scale eastward extrusion, clockwise rotation, and uplift, as indicated by its boundary deformation features. It was originally located beneath the narrowest part of the Qinling belt, where it underwent ultrahigh-pressure metamorphism in the Early Triassic.

Eastward migration of the Qaidam basin and its implications for Cenozoic evolution of the Altyn Tagh fault and associated river systems

The Qaidam basin is the largest Cenozoic intermontane basin within the Tibetan plateau, bounded by the Qilian Shan range to the north, the East Kunlun range to the south, the Altyn Tagh range to the west, and the Ela Shan range to the east. Its deposits consist of nonmarine sedimentary rocks, with a maximum thickness exceeding 13,400 m. Modern sedimentation mainly takes place along the marginal areas of the basin, where alluvial fans are formed along a series of rivers that originate from the surrounding mountains. Nevertheless, drilling, sedimentary, and seismic data reveal that the depocenter of the basin during much of Cenozoic time was not developed as foreland basins of the Qilian Shan belt to the north and the East Kunlun belt to the south, but instead the depocenter was in the center of the western part of the basin and had a northwest-southeast trend that shifted eastward ~380 km during Oligocene to Quaternary time. This indicates that most of the basin sediments were not derived from the surrounding mountain ranges. The results of the fi ssion-track age dating reveal that the rapid uplift of the Yousha Shan anticline bounding the depocenter to the southwest initiated at ca. 31 Ma, indicating that the southeastward shift of the depocenter began at that time, yielding a migration rate for the depocenter of 10.2 mm/yr. The northwestern margin of the depocenter of the Qaidam basin lies to the east of the westward prolongation of the Qaidam basin, the Tula trough, a 300-km-long narrow trough fl anked by the Altyn Tagh range along the Altyn Tagh strikeslip fault on the north and the East Kunlun thrust belt on the south. The sedimentary and deformational evidence from this region indicates that the Tula trough was formed as a syncline, along which a paleoriver fl owed to the east into the Qaidam basin. To the southwest, this river valley merges with the trace of the Altyn Tagh fault. The latter and its westward continuation are interpreted to have extended to the northern margin of the Pamir folded belt north of the western syntaxis of the Himalaya. We propose that most of the sediments within the depocenter of the Qaidam basin were transported from the northern margin of the Pamir folded belt along a progressively lengthening 2000-kmlong longitudinal river that developed along the left-lateral Altyn Tagh fault and its western continuation, as the eastward extrusion of the Tibetan plateau took place in Oligocene time. Both the crustal uplift and continuous southeastward aggradations of the sediments carried by the paleo‐Kunlun River are interpreted to be the cause for southeastward migration of the depocenter of the Qaidam basin. The longitudinal river disappeared in late Cenozoic time (2‐4 Ma) due to stream capture and climate change.

40Ar/39Ar thermochronological evidence for formation and Mesozoic evolution of the northern-central segment of the Altyn Tagh fault system in the northern Tibetan Plateau

To better constrain the probable timing of formation and evolution of the Altyn Tagh sinistral strike-slip system in the Mesozoic, a 40Ar/39Ar thermochronological study has been carried out in the north-central segment of the Altyn Tagh fault system, the northern margin of the Qaidam Basin, and the eastern Kunlun orogenic belt. Muscovite, biotite, and K-feldspar separated from mylonite, granite, pegmatite, and metamorphic rocks have been analyzed. The range of 40Ar/39Ar data and structural evidence indicate that a peak metamorphic event in terranes bordering the Altyn Tagh fault system occurred between 450 and 420 Ma. At ca. 250–230 Ma there is evidence for initial sinistral strike-slip shearing. Sinistral strike-slip deformation occurred later along the Altyn Tagh fault system at 165–160 Ma and 100–89 Ma, respectively.nnCooling histories in the northern margin of the Qaidam Basin and the eastern Kunlun orogenic belt show that these areas also experienced rapid cooling ca. 250–230 Ma, as was the case for the early Altyn Tagh fault system. This regional tectonic and cooling process indicates that the initial formation of the Altyn Tagh sinistral slip fault system occurred in latest Permian–Early Triassic time and was coupled with, or related to, suturing in the northern margin of the Qaidam Basin and the Kunlun orogenic belt. Cooling events along the Altyn Tagh fault system between 165 and 160 Ma and 100–89 Ma were accompanied by differential closure along the Bangong Lake–Nujiang suture zone in its eastern and western sectors during the Middle-Late Jurassic and Early Cretaceous, respectively ([Zhao et al., 2001][1]; [Wang et al., 2002][2]).nn [1]: #ref-48n [2]: #ref-41