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Cretaceous-Tertiary Shortening, Basin Development, and Volcanism in Central Tibet

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Cretaceous-Tertiary Shortening, Basin Development, and Volcanism in Central Tibet Cretaceous-Tertiary shortening, basin development, and volcanism in central Tibet Paul Kapp† Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA An Yin‡ T. Mark Harrison§ Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Lin Ding# Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, People’s Republic of China ABSTRACT Paleogene deformation in the Qiangtang INTRODUCTION terrane is characterized by widely distrib- The geologic map pattern of the Qiangtang uted, mainly north-dipping thrust faults There is growing geologic and geophysi- terrane in central Tibet defi nes a >600-km- that cut Eocene–Oligocene red beds and cal evidence for growth of the Tibetan plateau long and up to 270-km-wide east-plunging volcanic rocks in their footwall. Along the by northward underthrusting of India beneath structural culmination. It is characterized by Bangong suture, the north-dipping Shiqua- southern Tibet (e.g., Nelson et al., 1996; Owens early Mesozoic blueschist-bearing mélange nhe-Gaize-Amdo thrust system cuts 64 and and Zandt, 1997; DeCelles et al., 2002), lower and upper Paleozoic strata in the core and 43 m.y. old volcanic tuffs in its footwall and crustal fl ow (e.g., Royden et al., 1997; Clark and mainly Triassic–Jurassic strata along the accommodated >40 km of post–mid-Creta- Royden, 2000; Hodges et al., 2001; Ross et al., limbs. In the western Qiangtang terrane ceous shortening. The Tertiary south-dip- 2004), and major crustal shortening, basin infi ll- (~84°E), the culmination is unconformably ping Gaize–Siling Co backthrust bounds ing, and subduction of continental lithosphere overlain by weakly deformed mid-Creta- the southern margin of the Bangong suture in northern Tibet (e.g., Yin and Harrison, 2000; ceous volcanic fl ows and tuffs. Along the and marks the northernmost limit of mid- Tapponnier et al., 2001; Kind et al., 2002). The Bangong suture to the south (32°N, 84°E), Cretaceous marine strata in central Tibet. relative importance of these plateau-forming mid-Cretaceous nonmarine red beds and Cretaceous deformation and denudation processes in producing the vast, internally volcanic rocks lie unconformably on Jurassic in central Tibet is attributed to northward drained part of the plateau interior remains suture zone rocks. These relationships dem- underthrusting of the Lhasa terrane beneath poorly known. Also uncertain is the time inter- onstrate that west-central Tibet was above the Qiangtang terrane along the Bangong val over which the Tibetan plateau formed. A sea level during the mid-Cretaceous and suture. This model implies that (1) Creta- common assumption in most models is that the experienced signifi cant denudation prior to ceous strata along the Bangong suture and in thick crust (>65 km) and high elevation (~5 km) mid-Cretaceous time. Growth of the Qiang- the northern Lhasa terrane were deposited in of Tibet are dominantly Cenozoic features tang culmination is inferred to have initiated a fl exural foreland basin system and derived related to collision between India and Asia. The during southward emplacement of a thrust at least in part from the Qiangtang terrane, assumption that Tibet was near sea level prior to sheet of early Mesozoic mélange and upper and (2) the central Tibetan crust was thick- the Indo-Asian collision is based largely on the Paleozoic strata during the Early Cretaceous ened substantially prior to the Indo-Asian presence of marine strata as young as Eocene in Lhasa-Qiangtang collision. The north-south collision. Although its magnitude is poorly age along the Indus-Yarlung suture (Fig. 1) (e.g., width of the inferred thrust sheet provides a known, Tertiary shortening in the Qiangtang Burg and Chen, 1984; Searle et al., 1987, 1988) minimum slip estimate of ~150 km at 84°E, terrane is more prevalent than in the Lhasa and reports of Cretaceous to Paleocene marine decreasing eastward to ~70 km at 87°E. terrane; this difference may be attributed to limestones in the Lhasa terrane (summarized in the presence of underthrust mélange in the Zhang, 2000). While shallow marine limestones deeper central Tibetan crust, which would of Aptian–Albian age are well documented †E-mail: [email protected]. have made it weaker than the Lhasa terrane in the Lhasa terrane, the reports of younger ‡E-mail: [email protected]. during the Indo-Asian collision. marine strata have not been confi rmed. In fact, §Present address: Institute of Advanced Studies, post–mid-Cretaceous strata documented in the Australian National University, Canberra Australian Capital Territory, 0200, Australia; e-mail: mark.harr Keywords: Tibet, plateau formation, con- Lhasa terrane by other workers are entirely [email protected]. tinental collision, underthrusting, suture, nonmarine (Leeder et al., 1988; Yin et al., 1988; #E-mail: [email protected]. shortening. Murphy et al., 1997; Leier et al., 2002). Further- GSA Bulletin; July/August 2005; v. 117; no. 7/8; p. 865–878; doi: 10.1130/B25595.1; 8 fi gures; Data Repository item 2005099. For permission to copy, contact [email protected] © 2005 Geological Society of America 865 KAPP et al. Figure 1. Tectonic map of southern and central Tibet modifi ed from Kapp et al. (2003a). Tertiary thrust faults are shown in black. Major Mesozoic faults are shown in white: the Coqin thrust belt of Murphy et al. (1997) and early Mesozoic domal low-angle normal faults in the central Qiangtang terrane (Kapp et al., 2000, 2003b). Abbreviations: GCT—Great Counter thrust; GT—Gangdese thrust system; KF—Karakoram fault. more, post-Jurassic marine strata have not been scales of continental plateau formation, but also ment, and magmatism in central Tibet are inte- documented in large parts of the plateau north our ability to quantify continental deformation grated with those existing for the Lhasa terrane of the Lhasa terrane (Liu, 1988; Zhang, 2000). during the Indo-Asian collision. into a new tectonic model for crustal thickening Thus, it is possible that large parts of Tibet This paper presents the fi rst comprehensive and plateau growth. were substantially elevated prior to Indo-Asian overview of the Cretaceous–early Tertiary collision during Cretaceous–earliest Tertiary geology of a large (>85,000 km2) region in the REGIONAL GEOLOGIC BACKGROUND development of an Andean-style arc in southern middle of the Tibetan plateau (outlined area in Tibet (Fig. 1, Gangdese arc) and Lhasa–Qiang- Fig. 1). It is based on ~175 days of geologic The Qiangtang terrane (Fig. 1) exposes tang continental collision (Burg et al., 1983; mapping at 1:100,000 scale and geochrono- mainly Carboniferous and younger strata and England and Searle, 1986; Pan, 1993; Fielding, logic studies (details of methods available as early Mesozoic blueschist-bearing mélange 1996; Murphy et al., 1997; Yin and Harrison, supplementary information, see DR11) along of the central Qiangtang metamorphic belt 2000; Ding and Lai, 2003; Kapp et al., 2003a). the Bangong suture and two traverses across (Cheng and Xu, 1986; Liu, 1988; Kapp et al., Numerical models of Cenozoic orogeny in Asia the Qiangtang terrane (Fig. 2A).2 Our new data 2000; Kapp et al., 2003b). Cretaceous strata are that incorporate precollisional topography in bearing on the Cretaceous-Tertiary history of scarcely exposed, and the history of pre-Indo- Tibet make very different predictions regarding crustal shortening, denudation, basin develop- Asian collision shortening is poorly known the style, timing, and distribution of continen- (Cheng and Xu, 1986; Kidd et al., 1988; Leeder tal deformation from those that do not (e.g., et al., 1988; Yin et al., 1988). Paleogene strata England and Searle, 1986; Kong et al., 1997). 1GSA Data Repository item 2005099, geochrono- are nonmarine and crop out within relatively logic methods and Tables DR1 and DR2, is available on Therefore, the uncertainty in the crustal thicken- the Web at http://www.geosociety.org/pubs/ft2005.htm. narrow (<20-km-wide) east-trending basins ing history of the plateau interior limits not only Requests may also be sent to [email protected]. that locally contain 50–29 Ma potassic volcanic our understanding of the mechanisms and time 2Figure 2 is on an insert accompanying this issue. rocks (Cheng and Xu, 1986; Deng, 1989; Chung 866 Geological Society of America Bulletin, July/August 2005 CRETACEOUS-TERTIARY SHORTENING IN CENTRAL TIBET et al., 1998; Deng et al., 2000; Wang et al., 2001; (between ca. 121 and 99 Ma), and an upper unit of Cretaceous upper crustal shortening. In the Horton et al., 2002; Ding et al., 2003; Spurlin of fl uvial red beds (includes the Takena Forma- Lhasa area (Fig. 1), the Upper Cretaceous Tak- et al., 2005). Some exposures of Eocene-Oli- tion) (Leeder et al., 1988; Yin et al., 1988; Leier ena Formation was shortened by ~40% prior to gocene granitoids have also been documented et al., 2002; Zhang et al., 2004). It is debated Linzizong volcanism (Pan, 1993). in the northern Qiangtang terrane (Hacker et whether Cretaceous strata were deposited in al., 2000; Roger et al., 2000). Minimal Ter- an extensional back-arc (Zhang, 2000; Zhang CRETACEOUS SHORTENING, BASIN tiary upper-crustal shortening characterizes et al., 2004), intra-arc (Pan, 1993), retro-arc DEVELOPMENT, AND VOLCANISM the interior of the Qiangtang terrane along the foreland (England and Searle, 1986), or periph- Lhasa-Golmud Highway (Coward et al., 1988), eral foreland basin related to Lhasa-Qiangtang Qiangtang Terrane central Qiangtang metamorphic belt (Kapp et continental collision (Leeder et al., 1988; Yin et al., 2003b), and Yecheng-Shiquanhe Highway al., 1994; Kapp et al., 2003a). Between 80°E and 89°E, the regional geo- in far western Tibet (Fig.
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