Climate-driven environmental change in the Zhada basin, southwestern Tibetan Plateau Joel Saylor* Peter DeCelles Jay Quade Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT man, 1992; Molnar et al., 1993; France-Lanord tion the direct link between uplift and environ- and Derry, 1994; Ruddiman et al., 1997; An mental change on the Tibetan Plateau. The Zhada basin is a large Neogene et al., 2001; Abe et al., 2005; Molnar, 2005). The environmental effects of tectonics and extensional sag basin in the Tethyan Hima- Uplift is also thought to have directly driven climate change can best be addressed in basins laya of southwestern Tibet. In this paper environmental change on the Tibetan Plateau that contain all of the proxies mentioned above: we examine environmental changes in the (e.g., Liu, 1981a; Zhang et al., 1981; Zhu et pollen, leaf fossils, mammal fossils, and carbon- Zhada basin using sequence stratigraphy, al., 2004; Wang et al., 2006). However, recent ates used in stable isotope studies. A case in isotope stratigraphy, and lithostratigraphy. work suggests that global climate change point is the Zhada basin in southwestern Tibet. Sequence stratigraphy reveals a long-term drives climate and environmental change on However, the lack of a coherent, comprehensive tectonic signal in the formation and fi lling of the Tibetan Plateau (e.g., Dupont-Nivet et al., basin analysis integrating all the paleoenviron- the Zhada basin, as well as higher-frequency 2007). Moreover, uplift histories of the Tibetan mental proxies has hampered efforts to untangle cycles, which we attribute to Milankovitch Plateau based on faunal or fl oral associations the climatic and tectonic signals in the Zhada forcing. The record of Milankovitch cycles differ signifi cantly from those based on stable record. The Zhada Formation is described as in the Zhada basin implies that global cli- isotope and other quantitative paleoelevation both upward fi ning (Zhang et al., 1981; Zhou mate drove lake and wetland expansion and studies. Paleofl oral assemblages from Pleisto- et al., 2000; Li and Zhou, 2001b) and capped contraction in the southern Tibetan Pla- cene deposits on the Tibetan Plateau are simi- by boulder conglomerates (Zhu et al., 2004; teau from the Late Miocene to the Pleisto- lar to modern fl oral assemblages at low eleva- Zhu et al., 2007). There is similarly little con- cene. Sequence stratigraphy shows that the tions (e.g., Axelrod, 1981; Xu, 1981; Zhang et sensus regarding the basin’s tectonic origin. Zhada basin evolved from an overfi lled to al., 1981; Li and Zhou, 2001a, 2001b; Meng et The Zhada basin is presented as having devel- underfi lled basin, but continued evolution al., 2004; Molnar, 2005; Wang et al., 2006) and oped in the hanging wall of the low-angle South was truncated by an abrupt return to fl uvial are used to argue for plateau uplift of at least Tibetan detachment system or as a half-graben conditions. Isotope stratigraphy shows dis- 1 km since the Late Miocene. A similar argu- produced in response to arc-normal extension tinct drying cycles, particularly during times ment is based on the abundance of mammal (Wang et al., 2004; S.F. Wang et al., 2008a). It when the basin was underfi lled. megafauna on the Tibetan Plateau in the Late is also proposed to be a fl exural basin respond- A long-term environmental change Miocene–Pliocene and their relative paucity ing to arc-perpendicular compression (Zhou observed in the Zhada basin involves a now (e.g., Cao et al., 1981; Zhang et al., 1981; et al., 2000). The presence of capping boulder decrease in abundance of arboreal pollen Li and Li, 1990; Meng et al., 2004; Y. Wang et conglomerates has led to the suggestion that the in favor of nonarboreal pollen. The simi- al., 2008a). In contrast, other lines of evidence basin was recently uplifted (Zhu et al., 2004). larity between the long-term environmen- indicate that the southern Tibetan Plateau has Until recently, the Zhada basin was understood tal changes in the Zhada basin and those been at high elevations since at least the Mid- to have been at low elevations until as late as observed elsewhere on and around the dle Miocene (Garzione et al., 2000a; Rowley the Pleistocene (e.g., Zhang et al., 1981; Zhou et Tibetan Plateau suggests that those changes et al., 2001; Spicer et al., 2003; Currie et al., al., 2000; Li and Zhou, 2001a; Zhu et al., 2004). are due to global or regional climate change 2005; Saylor et al., 2009) and central Tibetan In a recent paper (Saylor et al., 2009) we rather than solely the result of uplift of the Plateau since at least the Oligocene (Cyr et al., documented the chronostratigraphy and stable Tibetan Plateau. 2005; Graham et al., 2005; Rowley and Cur- isotope record of the Zhada basin. Here we pro- rie, 2006; DeCelles et al., 2007; Dupont-Nivet vide basin-wide lithologic and sequence strati- INTRODUCTION et al., 2008). These paleoelevation studies also graphic correlations, frequency analysis of the show that uplift predated widespread Late record of environmental change, and a detailed Uplift of the Tibetan Plateau has long been Miocene climate change (see Molnar, 2005, isotope stratigraphy. Our results suggest that viewed as a major forcing factor in regional and for a summary of evidence for Late Miocene global climate change, possibly in conjunc- global climate change (e.g., Raymo and Ruddi- climate change). These studies call into ques- tion with regional climate change, controlled *Present address: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712-0254, USA. Geosphere; April 2010; v. 6; no. 2; p. 74–92; doi: 10.1130/GES00507.1; 12 fi gures; 1 table; 2 supplemental tables. 74 For permission to copy, contact [email protected] © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/2/74/3338188/74.pdf by guest on 23 September 2021 Sequence stratigraphy and climate cycles in southwestern Tibet 80°85° 90° 95° 100° 105° Quaternary Alluvium A 40° Abbreviations: 50550000 kmkm JSZ : Jinsha Suture Zone Zada Basin Fill t faul Tar imm gh BSZ : Bangong Suture Zone Kailas Conglomerate Ta BasinBasin ltyn ISZ : Indus Suture Zone A QaidamQaQa damddaam Mesozoic Tethyan rocks BasinBasB n MFT : Main Frontal thrust 35° Sonpan-GanziSonSoSoonnpanpapana -GGaGanzinzz TerraneTeTeerranerrar anene KF : Karakoram fault Paleozoic Tethyan rocks KFKF JSZJJSSZSZ QiangtangQiaQiQ angtgtangaanng TerraneTeTerrarrr anene Higher Himalayan rocks BSZ Gangdese Batholith ThTThisssS SStudytuttuddy Legend: 30° Thrust fault LhasaLLhLhahaasasa TerTTeTerraneere ranaane EZ Trace of measured section ISZSZS Detachment/normal fault H imalayan Thrust Belt North-south transect MFTT Strike-slip fault Great Counter Suture zone Northwest-southeast transect thrust B Leo Pargil Detachment Great Counter thrust Q Ayi Shan 1NWZ 2NWZ ZhadaZ 3NWZ Basin 32°00 Qusum Detachment ? 2NZ Karakoram Fault NRW System 1NZ NRE EZ 3NZ Indus-Yalu Suture Zone Guga 31°20 SZ SEZ Great Counter thrust China India S outh 30°40 11000000 k kmm Lake T Lake ib M e Pulan a ta i n Basin n D e Gurla Mandhata 79°00 C tac e hm 7728 m n en tra t l Thrust 30°00 India 80°00 Nepal 81°00 Figure 1. (A) Elevation, shaded relief, and generalized tectonic map of the Himalayan-Tibetan orogenic system showing the location of the Zhada basin relative to major structures. (B) Generalized geologic map of the Zhada region. Modifi ed from mapping by Cheng and Xu (1987), Murphy et al. (2000, 2002), and mapping by M. Murphy (2005, 2006, 2007, personal commun.). Geosphere, April 2010 75 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/2/74/3338188/74.pdf by guest on 23 September 2021 Saylor et al. environmental variability in the southwestern After deposition, the basin was incised to base- be physically traced (Saylor, 2008). Magneto- Tibetan Plateau during the Late Miocene– ment by the Sutlej River, exposing the complete stratigraphy linking the South Zhada, South- Pleistocene. The data also point to the possi- thickness of the Zhada Formation. The best east Zhada, and East Zhada sections provides bility of establishing a high-resolution climate estimate for the age of the Zhada Formation is additional constraints. A fi nal independent record for this high-elevation basin extending between ca. 9.2 and after 1 Ma, based on ver- constraint is the switch from exclusively C3 to from the Pleistocene to the Miocene. tebrate fossils and magnetostratigraphy (Fig. 2) mixed C3 and C4 vegetation that is observed (Lourens et al., 2004; S.F. Wang et al., 2008b; between 130 and 230 m in the South Zhada REGIONAL GEOLOGICAL SETTING Saylor et al., 2009). section and at ~300 m in the East Zhada sec- tion (Saylor et al., 2009). The expansion of C4 The Zhada basin is the largest late Cenozoic METHODS vegetation is observed across the Indian sub- sedimentary basin in the Himalaya. It is located continent and southern Tibet ca. 7 Ma (Quade just north of the high Himalayan ridge crest in Sedimentology et al., 1989, 1995; France-Lanord and Derry, the western part of the orogen (~32°N, 82°E; 1994; Garzione et al., 2000a; Ojha et al., 2000; Fig. 1A). The basin is at least 150 km long and We measured 14 stratigraphic sections span- Wang et al., 2006). 60 km wide, and the current outcrop extent of ning the basin extent from the Zhada county seat the basin fi ll is at least 9000 km2 (Fig. 1B). in the southeast to the Leo Pargil Range front Frequency Analysis of The Zhada basin is located in a zone of in the northwest (Fig.