Research Paper GEOSPHERE Along-strike diachroneity in deposition of the Kailas Formation in central southern Tibet: Implications for Indian slab dynamics GEOSPHERE; v. 12, no. 4 Ryan Leary*, Devon A. Orme*, Andrew K. Laskowski, Peter G. DeCelles, Paul Kapp, Barbara Carrapa, and Matt Dettinger Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA doi:10.1130/GES01325.1 15 figures; 1 table; 1 supplemental file ABSTRACT et al., 2015a, 2015b). In contrast, fewer studies have investigated the evolution of this area following initial collision (e.g., Ratschbacher et al., 1994; Wang et al., CORRESPONDENCE: ryan .leary@ nau .edu The Oligocene–Miocene Kailas Formation is exposed along strike for 2010; DeCelles et al., 2011; Carrapa et al., 2014; G. Li et al., 2015; S. Li et al., 2015), ~1300 km within the southernmost Lhasa terrane. In this study, we docu- and the geodynamics driving this evolution remain enigmatic. In particular, the CITATION: Leary, R., Orme, D.A., Laskowski, A.K., mented the sedimentology, structure, and age of this unit exposed between geodynamic origin of the thick (>4 km in some localities), continental Kailas For- DeCelles, P.G., Kapp, P., Carrapa, B., and Dettinger, M., 2016, Along-strike diachroneity in deposition of 87°E and 90°E. Within this region, the Kailas Formation is composed of conti- mation, deposited in the center of the largest Cenozoic continent-continent col- the Kailas Formation in central southern Tibet: Impli- nental deposits dominated by conglomerate and sandstone, with lesser vol- lision zone, is still debated (Aitchison et al., 2002, 2007; DeCelles et al., 2011; S. Li cations for Indian slab dynamics: Geosphere, v. 12, umes of siltstone and paleosols. These rocks were deposited nonconformably et al., 2015; Wang et al., 2015). However, more significant than the thickness of no. 4, p. 1198–1223, doi:10.1130/GES01325.1. on Gangdese Batholith and related volcanic rocks along their northern bound- these deposits is their along-strike continuity: The Kailas Formation is exposed, ary, whereas to the south, the south-dipping Great Counter Thrust places them with some variation in facies, for over 1300 km in Tibet and has been correlated Received 12 February 2016 Revision received 30 April 2016 in contact with Xigaze forearc and mélange units. We interpret the Kailas For- to rocks within the Indus Group several hundred kilometers farther west (Searle Accepted 16 June 2016 mation to have been deposited in alluvial-fan and fluvial environments with et al., 1990). Although initially thought to represent syncontractional deposits Published online 8 July 2016 sediment principally derived from the north. Based on sedimentology and related to a flexural basin associated with the late stage of India-Asia collision structural relationships, we interpret these rocks to have formed in a north- (Aitchison et al., 2007) or deformation related to the Great Counter Thrust (Yin south extensional setting. New zircon U-Pb ages from volcanic tuffs and flows et al., 1999; Wang et al., 2015), recent investigation in western Tibet (~81°E) indi- show that Kailas Formation deposition is younger to the east: Deposition oc- cates that these rocks were deposited in an extensional basin, possibly result- curred between 26 Ma and 24 Ma in western Tibet (81°E), at 25–23 Ma north of ing from Indian slab rollback (DeCelles et al., 2011). Precise dating of the Kailas Lazi (87.8°E), at 23–22 Ma near Dazhuka (89.8°E), and as late as 18 Ma south- Formation along the entire India-Asia suture zone is critical to understanding west of Lhasa (92°E). Overall, basin development propagated eastward at a the geodynamic drivers of collisional basin formation and can shed light on the rate of ~300 mm/yr. This pattern and rate of propagation are similar to that along-strike variations in collision zone evolution. In this study, we present the of the temporal-spatial distribution of adakitic and ultrapotassic magmatism results of field mapping, sedimentology, and stratigraphy, as well as new U-Pb within the Lhasa terrane to the north, which has been interpreted as a record zircon ages from interbedded tuffs and volcanic rocks for the Kailas Forma- of slab breakoff. Magmatism lags several million years behind Kailas basin de- tion preserved between 87°E and 90°E. We show that deposition of the Kailas velopment at most locations. We interpret the Kailas basin to have formed as Formation began in western Tibet around 26 Ma and became progressively the result of Indian slab shearing and breakoff, which began in western Tibet younger to the east, arriving at 90°E by 23 Ma. around 26 Ma and reached eastern Tibet by ca. 18 Ma. GEOLOGIC SETTING INTRODUCTION The Tibetan Plateau (Fig. 1) formed through the successive accretion of The rocks of the India-Asia suture zone preserve a record of processes re- continental fragments to the southern margin of Asia and the closure of their lated to the India-Asia collision and the subsequent evolution of southern Tibet. previously intervening oceanic basins (Allégre et al., 1984; Yin and Harrison, Much previous work has focused on the timing and nature of the India-Asia 2000). The southernmost of these continental fragments is the Lhasa terrane, collision (Besse et al., 1984; Garzanti et al., 1987; Leech et al., 2005; Green et al., which collided with Asia during Early Cretaceous time (Kapp et al., 2007). The 2008; Najman et al., 2010; van Hinsbergen et al., 2011; DeCelles et al., 2014; Hu appearance of Asian detritus in Tethyan Himalayan (Indian passive margin) strata beginning at ca. 60 Ma has been interpreted to represent the initiation of India-Asia collision after the Neo-Tethyan ocean basin was completely con- *Current addresses: Leary—School of Earth Sciences and Environmental Sustainability, North- For permission to copy, contact Copyright ern Arizona University, Flagstaff, Arizona 86011, USA; Orme—Department of Geological Sciences, sumed by northward subduction beneath the Lhasa terrane (Ding et al., 2005; Permissions, GSA, or [email protected]. Stanford University, Stanford, California 94305, USA. DeCelles et al., 2014; Orme et al., 2014; Hu et al., 2015b). The record of the © 2016 Geological Society of America GEOSPHERE | Volume 12 | Number 4 Leary et al. | Along-strike diachroneity in deposition of the Kailas Formation in central southern Tibet Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1198/4178139/1198.pdf 1198 by guest on 25 September 2021 Research Paper Qaidam Basin ATF 36°N Songpan-Ganzi Terrane AKMS SSZ ISZ 5 JSZ 34°N Qiangtang Terrane Karakoram Faul 32°N t 1 2 BNSZ Elevation (m) 4 5000 STDS Lhasa Terrane 4200 30°N 3300 4 2 a b c Lhasa 2400 Tethyan Himalay 3 2 1600 IASZ Fig. 2 Kailas Formation or Indus Group a 28°N Gangdese Magmatic Belt Linzizong Volcanics 500 km MBT 74°E 76°E 78°E 80°E 82°E 84°E 86°E 88°E 90°E 92°E 94°E 96°E Figure 1. Digital elevation model and simplified geologic map (after Yin and Harrison, 2000) of the Tibetan Plateau showing the Kailas Formation and Indus Group in black. Literature study loca- tions are numbered; locations from this study are lettered. Kailas basin: 1–4, a, b, c; Indus Basin: 5. Specific study locations: 1—Kailas Formation (DeCelles et al., 2011); 2—Gangdese conglomerate (Li et al., 2015); 3—Luobusa conglomerate (Kong et al., 2015); 4—Kailas Formation (Carrapa et al., 2014); 5—Indus Group (Henderson et al., 2010, 2011). IASZ—India-Asia suture zone; ISZ—Indus suture zone; SSZ—Shyok suture zone; BNSZ—Bangong-Nujian suture zone; STDS—South Tibetan detachment system; AKMS—Ayimaqin-Kunlun-Mutztagh suture zone; JSZ—Jinsha suture zone; ATF—Altyn Tagh fault; MBT—Main Boundary thrust. precollisional Asian margin, continental collision, and subsequent evolution magmatism continuing as late as 8 Ma (Zhu et al., 2011; Zhang et al., 2014); of the southern Tibetan Plateau is preserved in four major lithotectonic zones the highest apparent magmatic flux occurred ca. 52 Ma (Zhu et al., 2011). To within southern Tibet, Nepal, and northern India. From north to south, these the south of the Gangdese arc, the Xigaze forearc represents the remnants of are the Gangdese magmatic arc, the Xigaze forearc, the India-Asia suture zone, an originally much wider forearc basin that formed on the southern margin of and the Tethyan Himalayan thrust belt. The Gangdese magmatic arc formed Asia prior to collision (Einsele et al., 1994; Dürr et al., 1996; Wang et al., 2012; initially as an Andean-style arc produced by northward subduction of the Neo- Orme et al., 2014). Sedimentation in the Xigaze forearc basin began as early Tethyan slab beneath the Lhasa terrane, and it is dominated by calc-alkaline as Barremian time and continued until the early Eocene (51 Ma; Einsele et al., intrusive and volcanic rocks (e.g., He et al., 2007; Ji et al., 2009; Zhu et al., 2011). 1994; Dürr, 1996; Ziabrev et al., 2003; Orme et al., 2014; An et al., 2014; Hu et al., The arc was most active between ca. 200 Ma and 40 Ma, with low-volume 2015a; Huang et al., 2015). South of the forearc basin, the India-Asia suture zone GEOSPHERE | Volume 12 | Number 4 Leary et al. | Along-strike diachroneity in deposition of the Kailas Formation in central southern Tibet Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1198/4178139/1198.pdf 1199 by guest on 25 September 2021 Research Paper (IASZ on Fig. 1) is defined by ophiolitic slivers, radiolarian chert, and sedimen- cated ~30 km northeast of the town of Lazi (Fig. 3); the central locality, referred tary- and serpentinite-matrix mélange complexes (Cai et al., 2012; Huang et al., as the “Xigaze locality,” is exposed ~20 km northwest of the city of Xigaze 2015; Maffione et al., 2015; An et al., 2015).