The Ancestral Lhasa River: A Late Cretaceous trans-arc river that drained the proto–Tibetan Plateau

Abstract

Late Cretaceous trench basin strata were deposited in the subduction zone that consumed Neo-Tethyan oceanic lithosphere along the southern margin of the proto–Tibetan Plateau. We conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1716) collected near Dênggar, Tibet (∼500 km west of Lhasa), to assess the provenance of these rocks and reconstruct Late Cretaceous sediment transport pathways. They contained DZ ages that point to a unique source around Lhasa city, north of the Late Cretaceous Gangdese magmatic arc. The modern Lhasa River catchment contains the requisite sources, and its main trunk transects the Gangdese magmatic arc, joining with the Yarlung River at a barbed junction at the India-Asia suture. We infer that the Lhasa River is an ancient feature that transported sediment to the subduction zone in Late Cretaceous time and persisted during India-Asia collision. INTRODUCTION Rivers that drain the eastern Tibetan Plateau flow along intercontinental suture zones: the Yarlung along the India-Lhasa terrane suture, the Salween (Nagqu) along the Lhasa-Qiangtang suture, the Mekong along the Sibumasu-Indochina suture, and the Yangtze along the Qiangtang–Songpan-Ganzi suture (e.g., Brookfield, 1998; Zhang et al., 2019). Despite reshaping and/or reorganization (Burrard and Hayden, 1907; Brookfield, 1998; Clark et al., 2004; Clift et al., 2006; Zhang et al., 2012; Zhang et al., 2019) during India-Asia collision (Yin and Harrison, 2000), the rivers remained pinned to the low-lying suture zones for millions of years. This is intuitive, because suture zones mark the locations of former trenches and persist as lowlying features during intercontinental collision (e.g., Fielding et al., 1994). The Lhasa River is an exception. Its headwaters originate in the eastern portion of the Lhasa terrane, the southernmost of the crustal fragments that comprised Eurasia prior to collision with India. Its main trunk drains to the southwest across the Gangdese Mountains to the location where it meets the east-flowing Yarlung River at an abrupt junction with the acute angle on the downstream side (Fig. 1). This so-called barbed junction, along with lack of evidence for structural control along the trans-Gangdese segment (Harrison et al., 1992) and the observation of antecedent tributaries (Shackleton and Chang, 1988), suggests that the Lhasa River was established prior to tectonic uplift. Despite these geomorphological observations, geological evidence for the hypothesized ancestral Lhasa River is lacking. Prior to India-Asia collision, the southern Lhasa terrane was Andean-style (Murphy et al., 1997), comprising a Cretaceous to Paleocene subduction-accretion complex (e.g., Cai et al., 2012; Metcalf and Kapp, 2019) structurally overlying Jurassic and Cretaceous ophiolites (Göpel et al., 1984; McDermid et al., 2002; Hébert et al., 2012; Chan et al., 2015) and unconformably overlying Cretaceous to Paleocene Xigaze forearc basin strata (Einsele et al., 1994; Ding et al., 2005; Orme et al., 2015). These marginal assemblages are bounded to the north by the Late Triassic or Early Jurassic to Eocene Gangdese magmatic arc (Schärer et al., 1984; Lee et al., 2009; Zhu et al., 2011; Wang et al., 2016). Each feature developed in response to northward subduction of the Neo-Tethyan oceanic lithosphere beneath the southern Lhasa terrane as India converged on Eurasia (e.g., Shackleton, 1981; Tapponnier et al., 1981; Einsele et al., 1994; Cai et al., 2012; Orme and Laskowski, 2016; Metcalf and Kapp, 2017, 2019). Between 105 and 53 Ma, a retroarc fold-andthrust belt developed in the central Lhasa terrane that accommodated >55% shortening, creating a relatively high-elevation mountain belt analogous to the modern Andes and Late Cretaceous North American Cordillera (Kapp et al., 2007; Leier et al., 2007). Contemporaneous sedimentary rocks are the primary record of the development of the convergent margin, including the growth of a forearc basin between ca. 110 and 51 Ma (An et al., 2014; Orme et al., 2015; Orme and Laskowski, 2016) and sedimentation in trench basins prior to ca. 85–70 Ma incorporation into the subduction-accretion complex (Cai et al., 2012; An et al., 2018; Wang et al., 2018; Metcalf and Kapp, 2019). Given that Lhasa terrane crystallization ages and isotopic compositions of igneous rocks (Zhu et al., 2011; Wang et al., 2016; Chapman and Kapp, 2017) and characteristic age spectra of sedimentary rocks (Gehrels et al., 2011) are well known, detrital zircon (DZ) geochronology is appropriate for determining sediment provenance. In addition, DZ geochronology enables comparison of Late Cretaceous sediment transport with that of the modern Tibetan Plateau. If Late Cretaceous sediment transport pathways are analogous to those of the modern drainages, then the simplest interpretation is that the pathway is long-lived, and the Late Cretaceous equivalent is antecedent. We present data (n = 1716) from six DZ samples from Late Cretaceous (ca. 92–87 Ma) trench basin strata located near the town of Dênggar, ∼500 km west of Lhasa city (Fig. 1). We also compiled DZ data (N = 6, n = 1662) from intact trench basin strata in the Lazi region, ∼150 km to the east (Metcalf and Kapp, 2019). These data were compared with forearc basin strata to the north in the Saga (N = 18, n = 1577; Published online 6 September 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/11/1029/4852049/1029.pdf by Montana State University user on 20 November 2019 1030 www.gsapubs.org | Volume 47 | Number 11 | GEOLOGY | Geological Society of America Orme et al., 2015) and Lazi (N = 22, n = 2164; Orme and Laskowski, 2016) regions (Fig. 1). Our analysis reveals the distinct provenance of the trench basin and forearc basin strata, distinguished by the presence or absence of Late Triassic grains that were likely derived from the modern-day headwaters of the Lhasa River (Fig. 1). Based on these data, we infer that the trench basin is the ancient sedimentary record of the ancestral Lhasa River that transected the Late Cretaceous Gangdese magmatic arc and discharged into the Neo-Tethyan Ocean. METHODS Six samples were collected from fineto medium-grained feldspatholithic sandstones interbedded with green and black shale, red siliceous shale, and chert. Samples were collected within a stratigraphic section beginning at the base (sample 62518DA1_0) and ending 962 m stratigraphically higher (sample 62518DA1_962). The stratigraphic log is included in the GSA Data Repository1. Our mapping indicates that the section is located within at least 1 km of intact stratigraphy between two moderately north-dipping faults that we interpret as splays of the Zhongba-Gyangze thrust (Burg and Chen, 1984). We correlate these strata to the Rongmawa Formation (Cai et al., 2012; Wang et al., 2018; Metcalf and Kapp, 2019) on the basis of similar lithologies and structural position. DZ U-Pb ages were obtained for ∼300 zircon grains per sample using a Photon Machines Analyte G2 excimer laser attached to a Thermo Element2 high-resolution, singlecollector–inductively coupled plasma–mass spectrometer (HR-ICP-MS) at the Arizona LaserChron Center (University of Arizona, USA). Zircon targets were chosen randomly using the Crystal Site Selector program (creator: John H. Hartman, Department of Computer Science, University of Arizona, Tucson, Arizona, USA; [email protected].). Hf isotopic data were obtained for all zircon grains with U-Pb ages younger than 300 Ma in the stratigraphically lowest and highest samples (n = 50) to isolate non-Gondwanan igneous rocks using an identical laser-ablation (LA) system attached to a Nu Plasma multicollector ICP-MS. Sample information and all analytical data are reported in Tables DR1–DR3 in the Data Repository. Analysis and data reduction followed the methods of Gehrels and Pecha (2014) and Pullen et al. (2018). Kernel density estimate (KDE; Fig. 2) and multidimensional scaling (MDS; Fig. 3; Vermeesch, 2013) plots were generated using detritalPy (Sharman et al., 2018). Maximum depositional ages (MDAs) were calculated using three techniques: (1) age of the youngest single grain (YSG), (2) weighted mean age of the youngest two grains that overlapped at 1σ [YC1σ(2+)], and (3) weighted mean age of the youngest three grains that overlapped at 2σ ([YC2σ(3+)]; Table DR3). Source spectra (Fig. 2) were deconvolved using optimized nonnegative matrix factorization (NMF; Saylor et al., 2019), which uses an inverse approach to determine the optimum number of sources, reconstruct source spectra, and calculate weighting functions (Fig. 2). Provenance analysis was conducted by comparing trench basin DZ age distributions to the synthetic sources deconvolved from all trench basin samples using weighting functions (Fig. 2). Results from MDA and NMF calculations are reported in Tables DR4 and DR5. RESULTS The DZ age spectra from trench basin strata in the Dênggar region are remarkably internally consistent (Fig. 2). Furthermore, they are similar to trench basin samples near Lazi (Fig. 1), as evidenced by their proximity on the MDS plot (Fig. 3) and qualitative comparison of KDEs (Fig. 2). Both sample sets are characterized by prominent age-probability peaks at ca. 90 Ma, 120–130 Ma, 200–220 Ma, 500–550 Ma, and ca. 1200 Ma (Fig. 2). Lazi region samples have a higher relative abundance of ca. 90 Ma ages compared to Dênggar samples (Fig. 3). Forearc basin samples generally plot far from the trench basin samples on the MDS plot (Fig. 3), with separations between the Saga region and Lazi region samples (Figs. 1 and 2). We interpret these differences to preclude dominant reworking of forearc basin strata into the trench basin. A subset of Xigaze Group samples (N = 10) plots close to the trench basin samples (Fig. 3). These samples were divided by geographic location and combined to construct composite KDEs (group A, Fig. 2). The remaining samples compri