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The Ancestral Lhasa River: a Late Cretaceous Trans-Arc River That Drained the Proto–Tibetan Plateau Andrew K

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The Ancestral Lhasa River: a Late Cretaceous Trans-Arc River That Drained the Proto–Tibetan Plateau Andrew K https://doi.org/10.1130/G46823.1 Manuscript received 26 April 2019 Revised manuscript received 30 July 2019 Manuscript accepted 6 August 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 6 September 2019 The Ancestral Lhasa River: A Late Cretaceous trans-arc river that drained the proto–Tibetan Plateau Andrew K. Laskowski1, Devon A. Orme1, Fulong Cai2 and Lin Ding2 1 Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA 2 Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China ABSTRACT et al., 1994; Cai et al., 2012; Orme and Las- Late Cretaceous trench basin strata were deposited in the subduction zone that consumed kowski, 2016; Metcalf and Kapp, 2017, 2019). Neo-Tethyan oceanic lithosphere along the southern margin of the proto–Tibetan Plateau. We Between 105 and 53 Ma, a retroarc fold-and- conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1716) thrust belt developed in the central Lhasa terrane collected near Dênggar, Tibet (∼500 km west of Lhasa), to assess the provenance of these that accommodated >55% shortening, creating rocks and reconstruct Late Cretaceous sediment transport pathways. They contained DZ a relatively high-elevation mountain belt analo- ages that point to a unique source around Lhasa city, north of the Late Cretaceous Gangdese gous to the modern Andes and Late Cretaceous magmatic arc. The modern Lhasa River catchment contains the requisite sources, and its North American Cordillera (Kapp et al., 2007; main trunk transects the Gangdese magmatic arc, joining with the Yarlung River at a barbed Leier et al., 2007). Contemporaneous sedimen- junction at the India-Asia suture. We infer that the Lhasa River is an ancient feature that tary rocks are the primary record of the devel- transported sediment to the subduction zone in Late Cretaceous time and persisted during opment of the convergent margin, including India-Asia collision. the growth of a forearc basin between ca. 110 and 51 Ma (An et al., 2014; Orme et al., 2015; INTRODUCTION with lack of evidence for structural control along Orme and Laskowski, 2016) and sedimentation Rivers that drain the eastern Tibetan Plateau the trans-Gangdese segment (Harrison et al., in trench basins prior to ca. 85–70 Ma incorpora- flow along intercontinental suture zones: the 1992) and the observation of antecedent tributar- tion into the subduction-accretion complex (Cai Yarlung along the India-Lhasa terrane suture, ies (Shackleton and Chang, 1988), suggests that et al., 2012; An et al., 2018; Wang et al., 2018; the Salween (Nagqu) along the Lhasa-Qiangtang the Lhasa River was established prior to tectonic Metcalf and Kapp, 2019). Given that Lhasa ter- suture, the Mekong along the Sibumasu-Indo- uplift. Despite these geomorphological observa- rane crystallization ages and isotopic composi- china suture, and the Yangtze along the Qiang- tions, geological evidence for the hypothesized tions of igneous rocks (Zhu et al., 2011; Wang tang–Songpan-Ganzi suture (e.g., Brookfield, ancestral Lhasa River is lacking. et al., 2016; Chapman and Kapp, 2017) and 1998; Zhang et al., 2019). Despite reshaping Prior to India-Asia collision, the southern characteristic age spectra of sedimentary rocks and/or reorganization (Burrard and Hayden, Lhasa terrane was Andean-style (Murphy et al., (Gehrels et al., 2011) are well known, detrital 1907; Brookfield, 1998; Clark et al., 2004; Clift 1997), comprising a Cretaceous to Paleocene zircon (DZ) geochronology is appropriate for et al., 2006; Zhang et al., 2012; Zhang et al., subduction-accretion complex (e.g., Cai et al., determining sediment provenance. In addition, 2019) during India-Asia collision (Yin and Har- 2012; Metcalf and Kapp, 2019) structurally DZ geochronology enables comparison of Late rison, 2000), the rivers remained pinned to the overlying Jurassic and Cretaceous ophiolites Cretaceous sediment transport with that of the low-lying suture zones for millions of years. (Göpel et al., 1984; McDermid et al., 2002; modern Tibetan Plateau. If Late Cretaceous sedi- This is intuitive, because suture zones mark the Hébert et al., 2012; Chan et al., 2015) and un- ment transport pathways are analogous to those locations of former trenches and persist as low- conformably overlying Cretaceous to Paleocene of the modern drainages, then the simplest inter- lying features during intercontinental collision Xigaze forearc basin strata (Einsele et al., 1994; pretation is that the pathway is long-lived, and (e.g., Fielding et al., 1994). The Lhasa River Ding et al., 2005; Orme et al., 2015). These mar- the Late Cretaceous equivalent is antecedent. is an exception. Its headwaters originate in the ginal assemblages are bounded to the north by We present data (n = 1716) from six DZ eastern portion of the Lhasa terrane, the south- the Late Triassic or Early Jurassic to Eocene samples from Late Cretaceous (ca. 92–87 Ma) ernmost of the crustal fragments that comprised Gangdese magmatic arc (Schärer et al., 1984; trench basin strata located near the town of Eurasia prior to collision with India. Its main Lee et al., 2009; Zhu et al., 2011; Wang et al., Dênggar, ∼500 km west of Lhasa city (Fig. 1). trunk drains to the southwest across the Gang- 2016). Each feature developed in response to We also compiled DZ data (N = 6, n = 1662) dese Mountains to the location where it meets northward subduction of the Neo-Tethyan oce- from intact trench basin strata in the Lazi region, the east-flowing Yarlung River at an abrupt junc- anic lithosphere beneath the southern Lhasa ter- ∼150 km to the east (Metcalf and Kapp, 2019). tion with the acute angle on the downstream side rane as India converged on Eurasia (e.g., Shack- These data were compared with forearc basin (Fig. 1). This so-called barbed junction, along leton, 1981; Tapponnier et al., 1981; Einsele strata to the north in the Saga (N = 18, n = 1577; CITATION: Laskowski, A.K., Orme, D.A., Cai, F., and Ding, L., 2019, The Ancestral Lhasa River: A Late Cretaceous trans-arc river that drained the proto–Tibetan Plateau: Geology, v. 47, p. 1029–1033, https://doi.org/10.1130/G46823.1 Geological Society of America | GEOLOGY | Volume 47 | Number 11 | www.gsapubs.org 1029 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 84° E 86° E 88° E 90° E 92° E trench basin DZ age distributions to the synthetic QIANGTANG TERRANE sources deconvolved from all trench basin sam- Zagya Zangbo 32° N TIBETAN 32° N ples using weighting functions (Fig. 2). Results PLATEAU Siling Co N a gqu Zanggbbo from MDA and NMF calculations are reported o in Tables DR4 and DR5. angra Nam Tso T Yumc LHASA TERRANE Lhasa River Network RESULTS Orme et al., 2015 Luozha The DZ age spectra from trench basin strata 30° N N = 18 30° N Orme and Laskowski, 2016 Lhasa in the Dênggar region are remarkably internally Saga Yarlung River N = 22 Yarlung River consistent (Fig. 2). Furthermore, they are simi- New samples N = 6 Metcalf & Kapp, lar to trench basin samples near Lazi (Fig. 1), as 250 km Dênggar Lazi 2019; N = 6 TETHYAN HIMALAYA evidenced by their proximity on the MDS plot 84° E 86° E 88° E 90° E 92° E (Fig. 3) and qualitative comparison of KDEs Xigaze Forearc Basin Late Cretaceous-Paleocene Igneous Early Jurassic Igneous All other rocks (Fig. 2). Both sample sets are characterized by Subduction-Accretion L. Jurassic to E. Cretaceous Igneous Late Triassic Igneous Complex and ophiolites prominent age-probability peaks at ca. 90 Ma, 120–130 Ma, 200–220 Ma, 500–550 Ma, and Figure 1. Tectonic map of the Lhasa terrane and Yarlung suture zone in south-central Tibet, ca. 1200 Ma (Fig. 2). Lazi region samples have modified from Zhu et al. (2011). Mesozoic and early Cenozoic igneous rocks of the Lhasa terrane are highlighted due to their importance for detrital zircon provenance analysis. Xigaze Group a higher relative abundance of ca. 90 Ma ages forearc basin sample locations are shown in green (Orme et al., 2015; Orme and Laskowski, compared to Dênggar samples (Fig. 3). Forearc 2016), whereas Rongmawa Formation (trench basin) samples are shown in pink (this study; basin samples generally plot far from the trench Metcalf and Kapp, 2019). Major rivers and locations of towns were traced from the Esri World basin samples on the MDS plot (Fig. 3), with Topographic Map (http://arcg.is/0r4aaH). separations between the Saga region and Lazi region samples (Figs. 1 and 2). We interpret Orme et al., 2015) and Lazi (N = 22, n = 2164; DZ U-Pb ages were obtained for ∼300 these differences to preclude dominant rework- Orme and Laskowski, 2016) regions (Fig. 1). zircon grains per sample using a Photon ing of forearc basin strata into the trench ba- Our analysis reveals the distinct provenance of Machines Analyte G2 excimer laser attached sin. A subset of Xigaze Group samples (N = 10) the trench basin and forearc basin strata, dis- to a Thermo Element2 high-resolution, single- plots close to the trench basin samples (Fig. 3). tinguished by the presence or absence of Late collector–inductively coupled plasma–mass These samples were divided by geographic lo- Triassic grains that were likely derived from spectrometer (HR-ICP-MS) at the Arizona cation and combined to construct composite the modern-day headwaters of the Lhasa River LaserChron Center (University of Arizona, KDEs (group A, Fig. 2). The remaining samples (Fig. 1). Based on these data, we infer that the USA).
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