Age of Initiation of the India-Asia Collision in the East-Central Himalaya Bin Zhu, William S. F. Kidd, David B. Rowley,1 Brian S. Currie,2 and Naseer Shafique2 Department of Earth and Atmospheric Sciences, University at Albany, Albany, New York 12222, U.S.A. (e-mail: [email protected]) A B S T R A C T We document the stratigraphy and provenance of the lower Tertiary terrigenous sections in the Zhepure Shan region of the Tethyan Himalaya, southern Tibet, using petrographic and geochemical whole-rock and single-grain techniques. The Cretaceous–early Tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former Indian passive margin near the western end of the Zhepure Shan are conformably overlain by lower Tertiary clastic rocks. Sandstones in the Jidula Formation (Paleocene) mostly contain monocrystalline quartz grains of cratonic origin. In contrast, significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the Youxia (Early Eocene) and Shenkeza (post–Early Eocene) formations. Major, trace, and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the Jidula Formation consisted primarily of quartzose basement rocks, probably of Indian continental origin, whereas the sediments of the Youxia Formation were mainly derived from the uplifted Gangdese arc-trench system associated with the obduction of the Asian subduction complex. The compositions of Cr-rich spinels in the Youxia and Shenkeza sandstones resemble those from fore-arc peridotites and were most likely derived from the arc and ophiolite rocks along the developing Yarlung-Zangbo suture to the north. No spinels have been observed in the Jidula sandstones. Therefore, the early Tertiary detrital clastics in the Zhepure Shan record a marked change in provenance and sediment character and specifically at the time of deposition of the Youxia Formation, which contains a zone P-8 foram assemblage. This change indicates that the onset of India-Asia collision and the first development of the foreland (Ma in the both the western (Zanskar 0.2 ע basin immediately south of the India-Asia suture zone occurred at 50.6 and eastern (this study) Tethyan Himalaya. Online enhancements: appendix tables. Introduction The age of initiation of the India-Asia collision re- determining whether other events, including (1) mains a matter of considerable debate, with views changes in the India-Asia convergence rate between ranging from Late Cretaceous (165 Ma) to as young chrons 21 and 20 (Patriat and Achache 1984), (2) as 37 Ma (Rowley 1996, 1998, and references changes in India-Africa and India-Antarctica therein; de Sigoyer et al. 2000, 2001; Najman and spreading directions between chrons 18 and 20, and Garzanti 2000; Yin and Harrison 2000; Searle 2001; (3) changes in ocean chemistry, most notably Sr Najman et al. 2002). The precise timing of the start isotopes at about 42 Ma (Richter et al. 1992), are of collision between India and Asia is significant potentially linked with the onset of this collision. for estimating mass balance in the Himalayan sys- Sediment provenance in a foreland basin provides tem because of the high rate of India-Asia motion information on the tectonic evolution of the as- during the 65–47 Ma interval (Patriat and Achache sociated orogenic zone (Dickinson and Suczek 1984; Le Pichon et al. 1992; Rowley 1996) and for 1979; Zuffa 1980; Ingersoll et al. 1984; Dickinson 1985; Garzanti et al. 1996; Clingolani et al. 2003), Manuscript received June 4, 2004; accepted January 5, 2005. and it can constrain the age of the onset of collision 1 Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A. (Rowley and Kidd 1981). For example, detailed 2 Department of Geology, Miami University, Oxford, Ohio stratigraphic and petrographical analysis of the Cre- 45056, U.S.A. taceous to Eocene Tethyan sedimentary succession [The Journal of Geology, 2005, volume 113, p. 265–285] ᭧ 2005 by The University of Chicago. All rights reserved. 0022-1376/2005/11303-0002$15.00 265 266 B . Z H U E T A L . in the Zanskar region, NW Himalaya (Garzanti et al. 1987, 1996), indicates that the India-Asia col- lision started there in the late Ypresian ( 50.6 Ma). ∼ In this article, we report new data on the stratig- raphy, sandstone petrology, geochemical composi- tion, and spinel characteristics preserved in the lower Tertiary clastics exposed in the Zhepure Shan in the east-central Tethyan Himalaya of southern Tibet. Geological Framework The Tethyan Himalaya, located between the High Himalayan Crystalline belt to the south and the Indus-Yarlung-Zangbo Suture and the Lhasa block Figure 1. Simplified tectonic map of the east-central to the north (fig. 1), consist primarily of Late Pa- Tethyan Himalaya (modified after Willems et al. 1996). leozoic to Eocene marine sedimentary rocks, orig- The inset map shows the position of this area in the Himalayan system. MCT p Main Central Thrust; inally deposited along the northern passive conti- STDS p Southern Tibet Detachment system; C p nental margin of the Indian continent (Gansser Cholmolungma. 1964; Burg and Chen 1984). Deposition began with Late Paleozoic–Triassic rifting (Sengor et al. 1988; Sciunnach and Garzanti 1996; Garzanti 1999) dur- local stratotypes for the Cretaceous and lower Ter- ing the initial development of the Neo-Tethyan tiary in southern Tibet (Zhang and Geng 1983; Wil- Ocean, and a wide passive continental margin sub- lems et al. 1996). Six stratigraphic units (fig. 3) have sequently developed (Willems et al. 1996). During been defined in the Gongza section on the north the mid-Cretaceous, northward-directed subduc- slope of the Zhepure Shan mountain west of Shekar tion of the Neo-Tethyan oceanic crust beneath the Dzong (Willems et al. 1996), with ages at the top southern margin of Asia resulted in the develop- of the section modified on the basis of work re- ment of the Gangdise magmatic arc and Xigaze ported here. They are, from oldest to youngest, the fore-arc basin along the southern margin of the Gamba Group (late Albian–early Santonian), con- Lhasa block (Einsele et al. 1994; Durr 1996). sisting of marls and subordinate limestones; the The India-Asia collision has been argued to have Zhepure Shanbei Formation (early Santonian–mid- begun sometime in the Late Cretaceous–early Ter- dle Maastrichtian), comprising well-bedded lime- tiary interval, with the Indus-Yarlung-Zangbo su- stones interbedded with very thin layers of marl; ture marking the site of removal of Neo-Tethys oce- the Zhepure Shanpo Formation (middle Maas- anic lithosphere. The strata of the Tethyan trichtian–Early Paleocene), consisting of a lower in- Himalaya record the development and subsequent terval dominated by siliciclastic sandstones with closure of the Neo-Tethys and collision of India and minor calcareous sandstones and an upper se- Asia (Garzanti et al. 1987, 1996; Pivnik and Wells quence composed of pale-weathering, gray and 1996; Rowley 1996; Najman et al. 1997, 2001; Naj- black marlstone; the Jidula Formation (Danian) man and Garzanti 2000; Qayyum et al. 2001; Wan characterized by calcareous and glauconitic sand- et al. 2002; Wang et al. 2002). stones, shales, and mudstones; the Zhepure Shan Formation (late Danian–Ypresian), consisting of thick-bedded to massive limestones characterized Lithostratigraphy in the Zhepure Shan Region by abundant large foraminifera; and the uppermost This study concentrates on the well-exposed Ter- unit referred to by Willems et al. (1996) as the tiary clastic rocks resting conformably above shal- “Zongpubei Formation” (Ypresian or younger) of low marine carbonates and siliciclastics near the greenish-gray shales and some sandstones overlain western end of the Zhepure Shan (fig. 2). The Zhe- by red clay and siltstone with intercalations of pure Shan belongs to the southern continental sandstones (Wang et al. 2002). The Gamba Group shelf–dominated zone of the Tethyan Himalaya. through Zhepure Shan Formation all represent The Cretaceous to early Tertiary sequence of the shelf facies deposits of the Neo-Tethys and, in the southern Tethyan Himalayan zone is best exposed Zhepure Shan region, contain a relatively contin- in the ranges east of Gamba (Khampa Dzong) and uous depositional record from Albian through the west of Shekar Dzong (or New Tingri), regarded as Ypresian. On the basis of the work of Willems et Journal of Geology I N D I A - A S I A C O L L I S I O N 267 Figure 2. Simplified geologic map showing the location of the studied sections on the western end of the Zhepure Shan range. Interpretation extended from outcrop observations using Landsat TM image. al. (1996), the Zhepure Shan limestones are the the Zhepure Shan region were not studied in detail youngest well-dated marine sediments reported by Willems et al. (1996) because of poor exposure from the Tethyan Himalayas. and a fault contact between the unit and the un- Siliciclastic sediments, stratigraphically the derlying Zhepure Shan Formation in the section highest unit in the Zhepure Shan, were referred to they examined. A better-exposed 180-m-thick sec- as the Zongpubei Formation by Willems et al. tion termed the “Pengqu Formation” was described (1996). These were thought to be correlative with by Wang et al. (2002); they reported the location as siliciclastic sediments at the top of the Zhepure 5 km east of the Gongza section of Willems and Shan Formation in the Gamba region (180 km to Zhang (1993) and Willems et al. (1996). Wang et al. the ESE; fig. 1), named the Zongpubei Formation (2002) divided this section into the Enba Member by Willems et al. (1996). There are several reasons consisting of gray and yellowish-green shale inter- for questioning this correlation.