Discovery of the Longriba fault zone in eastern Bayan Har block, China and its tectonic implication (original) (raw)
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Tectonics
Global Positioning System (GPS) measurements across eastern Tibet reveal a sharp velocity gradient zone located about 150 km west of the Longmen Shan frontal thrust zone, where eastward block motion of Tibet decreases from~12 mm/yr to~3 mm/yr over a distance of less than 10 km. In order to investigate the tectonic cause for this rapid change in GPS velocity, together with systematic review on the available geological and geophysical data in easternmost Tibet, we provide new constraints on the tectonic feature of the Longriba fault zone from Advanced Land Observing Satellite Phased Array type L-band Synthetic Aperture Radar data. We propose that the NE striking Longriba fault zone is the key structure responsible for the observed sharp gradient in GPS velocities. In addition, the evidence indicates that the Longriba fault zone, instead of the Longmen Shan fault zone, marks the westernmost edge of the Yangtze crustal block. Given the irregular western margin of the Yangtze block, the Longriba fault zone represents part of the actual tectonic boundary between the Songpan-Ganzi terrane and the Yangtze block. The newly identified western edge of the Yangtze block implies a paleocontinent-ocean boundary at depth. This boundary was a potential weak zone and may have been exploited during the formation of the Longriba fault zone. The results of this paper should advance our understanding of the tectonic relationship between the Songpan-Ganzi terrane and Yangtze block and provide additional constraints for studies of the geodynamic response of eastern Tibet to the ongoing India-Eurasia collision.
Journal of Southeast Asian Earth Sciences
The Longmen Mountains are situated along the northeastern margin of the Tibetan Plateau, separating the Songpan-Ganze Fold Belt to the NW from the Sichuan Basin to the SE. During the Indosinian, about 200 Ma ago, the Songpan-Ganze Fold Belt was subjected to NE-SW compression which resulted in thrusts overprinted by NW-trending upright folds, whilst the adjacent Sichuan Basin remained undeformed. Differential strain between both terrains was accommodated across the Wenchuan-Mouwen Shear Zone, which represents the central axis of the Longmen Mountains and initially developed as a left-lateral strike-slip zone. Due to the constricted nature of the SE margin of the Songpan-Ganze Fold Belt, continued NE-SW shortening of the fold belt eventually caused NW-SE-directed compression along the Wenchuan-Mouwen Shear Zone resulting in localised crustal thickening and associated Barrovian-type amphibolite facies metamorphism. Thus the shear zone evolved from a left-lateral strike-slip zone into SE-directed thrust which accommodated uplift of the metamorphic terrains and the emplacement of a first generation of nappes onto the western margin of the Sichuan Basin. Final exhumation of the Longmen Mountains in combination with a second stage of nappe emplacement into the Sichuan Basin probably took place at the end of the Yenshanian. Himalayan deformational effects are restricted to the brittle faults and appear relatively unimportant. The large-scale, Indosinian, structural geometry of the Longmen Mountains is dominated by an en echelon arrangement of three major left-lateral shear zones that include the Wenchuan-Mouwen Shear Zone. The shears display a two dimensional Riedel array overlying a left-lateral basement fault. Medium-pressure amphibolitegrade metamorphic complexes occur at, or close to the shear terminations corresponding to zones of localised crustal thickening, whereas basement complexes occur in the overlaps of the shears corresponding to zones of local extension or pull-apart. The Longmen Mountains are essentially a left-lateral strike-slip zone in which crustal thickening and thrusting onto the Sichuan Basin are second order effects. Thus models which assume that the Longrnen Mountains represent the area where the Tibetan Plateau flows outward onto the Sichuan Basin are incorrect.
The Longriqu fault zone, eastern Tibetan Plateau: Segmentation and Holocene behavior
The dextral Longriba fault system (LFS), ~300 km long and constituting of two fault zones, has recently been recognized as an important structure of the eastern Tibetan plateau (Sichuan province), as it accommodates a significant amount of the deformation induced by the ongoing Indo-Asian collision. Although previous paleoseismological investigations highlighted its high seismogenic potential, no systematic quantification of the dextral displacements along the fault system has been undertaken so far. As such information is essential to appraise fault behavior, we propose here a first detailed analysis of the segmentation of the Longriqu fault, the northern fault zone of the LFS, and an offset inventory of morphological features along the fault, using high-resolution Pleiades satellite images. We identify six major segments forming a mature fault zone. Offsets inventory suggests a characteristic coseismic displacement of ~4 m. Two alluvial fans, with minimum ages of 6.7 and 13.2 ka, respectively displaced by 23 ± 7 m and 40 ± 5 m, give an estimate of the maximal horizontal slip rate on the Longriqu fault of 3.2 ± 1.1 mm yr À1. As a result, a minimum ~1340 year time interval between earthquakes is expected.
Tectonophysics, 2013
The 12 May 2008 Wenchuan earthquake (M w 7.9) generated a 285-km-long surface rupture zone along the Longmen Shan fold-and-thrust belt (LSFTB) on the eastern margin of Tibetan Plateau. The Wenchuan earthquake did not rupture into the southwestern Longmen Shan, along which there is no evidence for large paleo-or historical earthquakes. Seismic reflection profiles and field investigations reveal that the 50-km-long Qiongxi thrust fault (QTF) of the southern LSFTB is currently active. The QTF consists of three west-dipping ramp segments overlain by fault-bend folds rooted in a regional detachment that transfers shortening from the mountain belt into the Sichuan basin. Trench investigations, coupled with interpretations of seismic reflection profiles and radiocarbon results, show that a recent surface-rupturing earthquake occurred on the QTF during the Late Ming to Qing Dynasty, between AD 1600 and 1800. In addition, seismic reflection profile and topographic analysis indicate the presence of a subtle topographic, produced by kink-band migration folding above a fault bend at about 5 km depth. These findings confirm that the QTF is a significant seismic hazard, and that it should be incorporated into current regional seismic hazard models for the densely populated Sichuan basin.
Following the 2008 Wenchuan earthquake (Sichuan, China), the dextral strike-slip Longriba fault system (LFS) has been recognized as a main intracontinental structural boundary within the eastern Tibetan Plateau. While numerous studies have focused on the Longmen Shan frontal range to constrain the dynamics of the eastern Tibetan margin, little is known on the LFS, particularly on its eventual influence on the geomorphological evolution of the latter. Here, we provide a new data set of denudation rates derived from beryllium-10 concentrations in river sediments from 33 medium-sized catchments. Our sampling area covers the frontier between the dissected margin and the low relief interior plateau. Our results reveal a sharp increase of denudation across the LFS, from <0.1 mm/y in the Ruoergai basin to 0.3 mm/y toward the Longmen Shan range. Such denudation pattern indicates a major morphotectonic control of the fault system on the eastern Tibetan margin evolution. Additional topographic analysis confirms the role of the LFS as an important geomorphological boundary, restraining the westward propagation of river incision into the low-relief areas, thus partly preventing the dismantling of the eastern Tibetan Plateau.
Tectonics, 1991
The Haiyuan area, located along the northeastern margin of the Tibetan Plateau in north central China, provides a laboratory for studying how the plateau has grown in late Cenozoic time. Rocks in the area range from pre-Silurian (Precambrian?) to Recent; the pre-Silurian and Cenozoic rocks form the most extensive outcrops. The pre-Silurian rocks consist of amphibolite-and greenschist-grade metasedimentary and metaigneous rocks unconformably overlain by Silurian and Devonian red beds. All of these rocks are intruded by granodiorite of unknown age. Cenozoic rocks consist of 2.6-3.0 km of Eocene to Miocene red beds that were deposited over an extensive area in this part of China. Pliocene conglomerate contains clasts from all older formations and is interpreted to have been derived from highlands developed during the beginning of Cenozoic deformation in the Haiyuan area. Except for the widespread 1oess deposits, Quaternary rocks reflect deposition in local tectonic environments. The oldest Cenozoic structures in the Haiyuan area are folds and small thrust faults that generally strike N30ø-45øW and involve mostly pre-Quaternary rocks. These structures and all the Quaternary rocks are cut by the Haiyuan left-lateral strike-slip (left-slip) fault zone that generally trends N60ø-65øW and is nearly vertical. At the western end of the mapped area a fault zone, which strikes N75ø-90øW, forms a left-stepping transfer zone that connects with another segment of the Haiyuan fault zone, which continues N60ø-65øW west into Gansu Province. A small basin, the Salt Lake Basin, is marked by active faults in the area of the transfer zone and is interpreted as a pull-apart basin along the left-slip Haiyuan fault zone. At its eastern end the Haiyuan fault zone has an irregular surface trace; east of Luzigou an active fault striking N35ø-45øW branches to the south. This southern branch appears to be a younger fault and now accommodates most of the left-slip deformation that formerly occurred on the easternmost part of the Haiyuan fault zone. This younger fault connects through a left-stepping transfer zone to a parallel fault, the Xiaokou fault, that can be traced into the Liupan Shan about 60 km to the southeast. The Laohuyaoxian Basin is interpreted as a very young pull-apart basin in the area of the transfer zone. Matching different geological features across the Haiyuan fault zone yields a total left-slip offset of between 10.5 and 15.5 km, and the best constrained offsets yield 12.9-14.8 km. If left slip began near the end of the Pliocene time or earliest Pleistocene time, it indicates an average slip rate between 5 and 10 mm/yr. Progressively smaller offsets can be determined on progressively younger geological features, but dates for these younger features are too imprecise to constrain slip rates through time. Surface ruptures that formed during the 1920 Haiyuan earthquake (M = 8.7) show mostly left-slip displacement with magnitudes of more than 10 m in some places. Active faulting in the region suggests the Tibetan Plateau may be extending to the northeast in time. In the Haiyuan area, deformation probably began in Pliocene time, compared with a likely earlier initiation to the southwest; thus deformation began about 40-45 m.y. after collision between India and Asia. Formation of the low ranges to the northeast of the Haiyuan area, however, may have developed at different times and deformation may not have propagated regularly to the northeast. A total displacement of 10.5-15.5 km on the Haiyuan fault zone indicates that this fault zone does not accommodate large-scale eastward lateral transfer of continental fragments in the northern Tibetan Plateau.
Tectonics
The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U-Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10-15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25-35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the region's deformation, and in both above-mentioned deformation models.
Lithosphere, 2016
The Yabrai range-front fault accommodates deformation within the middle Gobi Alashan block between the Tibetan Plateau and the Ordos block. As such, it provides the opportunity to examine the transition between contractional deformation associated with the growth of the Tibetan Plateau and extensional deformation across North China. Geomorphic mapping of the active fault trace and trench investigations reveal that the Yabrai range-front fault is composed of three segments of varying fault strike, but for which the sense of motion, scarp height, and slip history appear to be kinematically compatible along the fault. Displaced Holocene and late Pleistocene alluvial deposits indicate that the southwestern segment is characterized by oblique-normal displacement with a minor sinistral component, whereas the middle segment appears to exhibit nearly dip-slip normal displacement. In contrast, slip along the northeastern segment appears to be primarily sinistral strike-slip with a minor reverse component. Geomorphically fresh fault scarps are developed within late Pleistocene-Holocene alluvial fans and terraces along the southwestern and northeastern segments, whereas the middle segment of the fault defines the bedrock-alluvial contact along the range front. The 10 Be exposure ages of displaced alluvial fans along the southwestern segment yield a throw rate of ~0.1 mm/yr over late Pleistocene time. Lateral slip rates along the northeastern fault segment range between 0.23 ± 0.02 and 0.78 ± 0.12 mm/yr. Regionally, the orientation and sense of motion along the Yabrai range-front fault are consistent with NE-SW shortening, and we suggest that recent activity along this fault system reflects incipient deformation of the foreland at the northeastern margin of the Tibetan Plateau.
Preface to the special issue on Structure and dynamics of the Longmenshan fault zone
Journal of Asian Earth Sciences, 2020
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Crustal deformation along the Altyn Tagh fault system, western China, from GPS
Journal of Geophysical Research, 2001
We collected GPS data from the southern Tarim basin, the Qaidam basin, and the western Kunlun Shan region between 1993 and 1998 to determine crustal deformation along the Altyn Tagh fault system at the northern margin of the Tibetan plateau. We conclude from these data that the Altyn Tagh is a left-lateral strike slip fault with a current slip rate of ˜9 mm/yr, in sharp contrast with geological estimates of 20-30 mm/yr. This contrast poses an enigma: because the GPS data cover a wider region than the geologic data, they might be expected to reveal somewhat more slip. We also find that the Tarim and Qaidam basins behave as rigid blocks within the uncertainty of our measurements, rotating clockwise at a rate of ˜11 and ˜4.5 nrad/yr, respectively, with respect to the Eurasia plate. The rotation of the Tarim basin causes convergence across the Tian Shan, increasing progressively westward from ˜6 mm/yr at 87°E to ˜18 mm/yr at 77°E. Our data and other GPS data suggest that the Indo-Asia collision is mainly accommodated by crustal shortening along the main Himalayan thrust system (˜53%) and the Tian Shan contractional belt (˜19%). Eastward extrusion of the Tibetan plateau along the Altyn Tagh and Kunlun faults accommodates only ˜23% of the Indo-Asia convergence.