TECTONICS, VOL. 21, NO. 1, 10.1029/2000TC001246, 2002
[10] The eastern margin of the Tibetan Plateau in the vicinity of the Sichuan Basin coincides with the Longmen Shan thrust belt, a feature developed along the western margin of the Yangtze craton in Mesozoic time and reactivated in the Cenozoic [Dirks et al., 1994; Burchfiel et al., 1995]. Although the focus of this paper is on the Cenozoic thermal evolution of rocks along the eastern margin, our results have important implications for the Mesozoic tectonic history of this region as well. In this section we briefly summarize salient aspects of the geology of the Longmen Shan region, discuss the Mesozoic tectonic history, and provide an overview of the Cenozoic tectonics. For a more complete treatment of the geology of this complex region, see Burchfiel et al. [1995].
3.1. Geology of the Longmen Shan
[11] For simplicity of discussion we divide the geology of the Longmen Shan region into four distinct tectonostratigraphic packages (Figure 2). From oldest to youngest, these include the following: (1) crystalline basement rocks of the Yangtze craton, (2) Neoproterozoic-Permian passive margin sediments, (3) Triassic flysch of the Songpan-Garze terrane, and (4) Mesozoic-Cenozoic terrestrial sediments in the Sichuan Basin. These packages are juxtaposed across a series of structures with polyphase histories, collectively termed the Longmen Shan Thrust Belt [Dirks et al., 1994; Burchfiel et al., 1995]. We describe each of these components and their structural relations in this section.
[12] Crystalline rocks of the Yangtze craton are exposed in a belt of basement massifs adjacent to the southwest corner of the Sichuan Basin (Figure 2). The massifs consist of quartzofeldspathic gneisses and associated granitoids that are thought to be Precambrian [Ministry of Geology and Mineral Resources, 1991]. They are structurally overlain by a parautochthonous passive margin sequence, and thus assumed to represent the basement of the Yangtze craton. The passive margin sequence itself consists primarily of shallow water marine rocks of Neoproterozoic (Sinian) to Permian age. Permian carbonates generally grade into deep-water deposits of the Songpan-Garze terrane (Figure 2), a ~500,000 km2 region of vertically dipping, isoclinally folded Triassic flysch and graywacke that may represent incomplete closure of a Paleotethyan ocean basin [Sengor et al., 1993; Zhou and Graham, 1996].
[13] All three tectonostratigraphic packages were deformed during Mesozoic time as evident in a series of east directed thrusts that place the Neoproterozoic-Triassic passive margin sequence atop the craton [Chen and Chen, 1987; Chen et al., 1994a; Dirks et al., 1994; Burchfiel et al., 1995; Chen and Wilson, 1996]. The thrust belt trends northeast and merges with the E-W trending Qinling orogen [Mattauer et al., 1985] north of the Sichuan Basin. Northwest trending folds within the Songpan-Garze wrap into parallelism with the western edge of the Longmen Shan thrust belt (Figure 2), and likely indicate that the thrust belt had a significant sinistral component during Mesozoic transpressional deformation [Dirks et al., 1994; Burchfiel et al., 1995].
[14] Substantial metamorphism and plutonism accompanied Mesozoic deformation in the Longmen Shan region. Metamorphic grade of the passive margin sequence ranges from negligible to lower amphibolite and generally increases toward the hinterland [Dirks et al., 1994; Burchfiel et al., 1995]. Metamorphism was accompanied by emplacement of a suite of Mesozoic plutons (generally granodiorite to monzonite) into the Triassic flysch northwest of the thrust belt [Roger et al., 1995b]. These plutons are generally macroscopically undeformed and cut across isoclinal, upright folds within the flysch. Contact metamorphic mineral assemblages from the flysch adjacent to the plutons include andalusite and cordierite [Dirks et al., 1994], suggesting that the plutons were emplaced at pressures <2.5–3 kbar [Bohlen et al., 1991]. Contact metamorphic minerals overgrow regional fabrics within the flysch, and, combined with the undeformed nature of the granitoids themselves, suggest that most of the plutons were emplaced late in the deformational history [Dirks et al., 1994].
[15] The fourth tectonostratigraphic package in the region consists of terrestrial sediments east of the thrust belt. Triassic-Cretaceous sediments comprise westward thickening wedges of fluvial mudstones, sandstones, and conglomerates deposited in an flexural basin in front of the thrust belt [Chen et al., 1994a; Burchfiel et al., 1995]. Total thickness of these deposits locally exceeds 10 km [Burchfiel et al., 1995]. Cenozoic sediments, however, are restricted to the southwest corner of the basin and consist of a thin (<700 m) sequence of terrestrial fluvial and lacustrine deposits.
3.2. Mesozoic Tectonics of the Longmen Shan
[16] Mesozoic deformation in the Longmen Shan thrust belt is reflected in major changes in depositional patterns along the western margin of the Sichuan Basin. Lower and Middle Triassic rocks atop the Yangtze craton are shallow marine to nonmarine in character. Rapid deposition of terrestrial deposits (up to several kilometers) began in the Late Triassic [Burchfiel et al., 1995]. East directed thrust sheets in the Longmen Shan (Tangwangzhai nappe, Figure 2) carry Paleozoic passive margin rocks above uppermost Triassic and Lower Jurassic terrestrial sediments in the Sichuan Basin [Chen et al., 1994a]. In several localities east of the Min Shan (Figure 2) these faults are overlapped by Early and Middle Jurassic basin fill that narrowly restricts the timing of deformation along the thrust front [Burchfiel et al., 1995].
[17] Within the Songpan-Garze terrane to the west, the Lower and Middle Triassic is characterized by deep marine deposition, with a rapid influx of turbidites and flysch beginning in the Middle Triassic [Burchfiel et al., 1995]. By Late Triassic time, the basin had shallowed, and coal-bearing clastics were deposited locally. Although all of these units were severely deformed and imbricated during basin closure in the latest Triassic [Zhou and Graham, 1996], the preservation of this uppermost basin fill suggests that the Mesozoic tectonism within the Songpan-Garze did not create high mountains subject to significant erosion [Burchfiel et al., 1995].
[18] There is little direct evidence for late Middle Jurassic to Cretaceous deformation within the Longmen Shan region, although westward thickening wedges of conglomerates of this age indicate continued subsidence along the foredeep [Burchfiel et al., 1995]. Arne et al. [1997] obtained Late Cretaceous 40Ar/39Ar ages (circa 120–130 Ma) from muscovites in metamorphic rocks northwest of the Wenchuan-Maowen fault. Although they interpreted these ages as reflecting differential cooling across this shear zone, the extent of Late Cretaceous deformation in the Longmen Shan remains essentially unknown.
3.3. Cenozoic Tectonics of the Longmen Shan
[19] Despite the impressive topographic front along the Longmen Shan, shortening across this margin of the plateau appears to have been relatively minor during Cenozoic time. The lack of a Cenozoic foredeep along in the Sichuan Basin indicates that flexural loading of the basin was negligible during formation of the plateau [Burchfiel et al., 1995; Royden et al., 1997]. In addition, space geodetic studies indicate that active shortening across the Longmen Shan is less than a few mm/yr and within uncertainty of zero [King et al., 1997; Chen et al., 2000]. Significant tilting of Pleistocene-Recent markers is present within the western Min Shan, a N-S trending range along the margin of the plateau north and west of the Sichuan Basin (Figure 2) [Tang et al., 1993; Chen et al., 1994b; Kirby et al., 2000], yet it occurs in the absence of resolvable E-W shortening between the plateau and the basin [Kirby et al., 2000].
[20] Some Cenozoic deformation is present in the southern Longmen Shan and has important implications for the timing of tectonism along this margin of the plateau. In the southwestern Sichuan Basin, a series of NE trending folds involve Cretaceous-Oligocene rocks and are overlapped by flat-lying Neogene sediments (Figure 2). A belt of Paleozoic klippen was emplaced above folds in Upper Jurassic rocks that are continuous with the rocks below the Cenozoic section [Burchfiel et al., 1995]. These relationships indicate that some shortening occurred after the Oligocene, but the exact timing is unknown. Facies relationships between the klippen and units further west suggest that Cenozoic shortening in the southern Longmen Shan region is less than a few tens of kilometers [Burchfiel et al., 1995].
[21] The degree of Cenozoic deformation west of the klippen is unknown, although shortening appears to be relatively minor [Dirks et al., 1994; Burchfiel et al., 1995]. Interestingly, a belt of normal faults occurs along the western margin of the Precambrian massifs [Burchfiel et al., 1995]. These faults are characterized by low-grade mylonitic textures with kinematic indicators that suggest west side down displacement. These faults may be continuous with similar structures along the thrust belt to the southwest (Figure 2). 40Ar/39Ar cooling ages from micas within these mylonites range from Cretaceous to mid-Tertiary (circa 25–30 Ma) [Hames and Burchfiel, 1993], and the authors interpreted the youngest of these ages as a best estimate of the timing of normal faulting.
[22] Arne et al. [1997] interpreted fission track age variations in zircon and apatite across the Longmen Shan thrust belt as indicating differential cooling in response to Cenozoic reactivation of Mesozoic structures. Zircon fission track ages west of the Wenchuan-Maowen fault range from 38 to 68 Ma, while a single sample east of the fault yielded an age of 110 Ma. Apatite fission track ages are typically late Miocene within the thrust belt and appear to be invariant across the Wenchuan-Maowen fault [Arne et al., 1997] (Figure 2). Apatites from Mesozoic terrestrial deposits in the Sichuan Basin yield fission track ages between 93 and 189 Ma, in all cases near the depositional age of the sediments [Arne et al., 1997]. In the Min Shan region, apatites yielded ages ranging from circa 120 to 70 Ma. Track length models from these three samples suggest an increase in cooling rates sometime after circa 20 Ma, that the authors interpret to reflect the inception of deformation in this region of the plateau [Arne et al., 1997].
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