Geoscience Frontiers 9 (2018) 591e594

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Field report: Research along the Yarlung Suture Zone in Southern Tibet,a persistent geological frontier

a r t i c l e i n f o

Article history:Received 14 September 2017Received in revised form 29 September 2017Accepted 3 November 2017Available online 14 November 2017Handling Editor: Christopher J. Spencer

Keywords:Tibetan PlateauHimalayaField workStructural geologySuture zoneHP metamorphismUHP metamorphismHistory of geology

Peer-review under responsibility of China University

a b s t r a c t

The Yarlung Suture Zone in Southern Tibet marks the boundary between India and Asiaeformerlyseparated by an ocean basineand is a critical record of the tectonic processes that created the TibetanPlateau. The Yarlung Suture Zone is also a frontier research area, as difficulty of access has limitedresearch activity, providing ample opportunities for new discoveries. This paper documents fieldresearch conducted by the authors along the Yarlung suture zone in eastern Xigaze (Shigatse, Rikaze)County, w250 km west of the city of Lhasa, in July 2017. The goal of this research was to map the SutureZone structure in detail, and more specifically to understand the branching relationships between twomajor fault systemsdthe Great Counter Thrust and Gangdese Thrust. A summary of early geologicalexploration is included to provide context for this research.

� 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting byElsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

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1. Introduction

The Tibetan Plateau (Fig. 1) comprises a spectacular mountainbelt of extreme elevation (average >4500 m) and aerial extent(2.5 million km2; Fig. 1). The rugged beauty of this region and thepotential it holds for scientific discovery motivated European expe-ditions beginning in the late 19th century. Today, the region con-tinues to attract the attention of the international scientificcommunity. The Tibetan Plateau is alluring to geologists studyingcontinental tectonic processes, as it was produced by a series ofinter-continental collisions that culminated in the ongoing India-Asia collision. Despite sustained interest, the challenge of accessingthe Tibetan Plateau limits research activity, maintaining its statusas a research frontier. In this paper, I document field structural ge-ology research conducted in July 2017 along the geological bound-ary between India and Asia in southern Tibetethe Yarlung SutureZone (Fig. 1). Major geological expeditions in Tibet and modern de-velopments are summarized to provide historical context. My per-sonal experiences with travel, permitting, and field work arehighlighted to reveal the ways that the modern geological expedi-tion has evolved from the time of the early explorers, presentingnew challenges and opportunities.

2. Geological exploration of the Tibetan Plateau

Systematic exploration of the Tibetan Plateau by western scien-tists began with the expeditions of Swedish geographer Sven

of Geosciences (Beijing).

Hedin, who mapped southern Tibet and located the source of theBrahmaputra and Indus Rivers at the turn of the 20th century. Earlygeological exploration began about thirty years later, primarilyfocusing on the Himalaya in India, Nepal, and Pakistan due to thedifficulty of accessing the Plateau’s interior. Augusto Gansser, aSwiss geologist, discovered fault-bounded slivers of oceanic crust(ophiolites) along the Indus River in northwestern India (Heimand Gansser, 1939), providing evidence that would become criticalduring the development of Plate Tectonic Theory. In the early1970’s, John Dewey and Kevin Burke became the first to interpretthe Tibetan Plateau as the result of collision between India andAsia (e.g. Dewey and Burke, 1973). Their paper revisited samplesof volcanic rocks collected by Sven Hedin in 1916, arguing thatthey provided evidence for subduction of oceanic crust prior toIndia-Asia collision (Harris, 1992).

The first large geological expedition to Tibet took place between1980 and 1983, involving cooperation between French and Chinesescientists to map and document more than 2000 km of the Indus-Yarlung Suture Zone (Tapponnier et al., 1981). Soon after, the 1985Geotraverse expedition documented more than 1700 km of the Ti-betan Plateau interior between the city of Lhasa in southern Tibetand the town of Golmud (Fig. 1). Their 22-person team identifiedseveral distinct terranes (Fig. 1) that accreted to the southernmargin of Asia during Paleozoic and Mesozoic time, culminatingwith Cenozoic India-Asia collision (Chengfa et al., 1986). The num-ber of geologists working on the Tibetan Plateau increased dramat-ically after 1990, likely resulting from a combination of lessrestrictive entry policies and the increasing ease of internationaltravel.

Figure 1. (A) Geographic and tectonic features of the Tibetan Plateau. Sutures are indicated by the dashed line. Locations discussed in the text are indicated by the white circles(major cities), black circles (minor cities), diamonds (high-pressure metamorphism localities), and airplane (airport) symbols. Digital elevation data from the Global Multi-Resolution Topography Database (gmrt.marine-geo.org). (B) Generalized geologic cross section from the southern Lhasa Terrane to the Indian foreland, through the Lopu Range,adapted from Laskowski et al. (2016). Cgl. e nonmarine conglomeratic strata, STDS e South Tibetan Detachment System, MCT e Main Central Thrust, MBT e Main Boundary Thrust,GCT e Great Counter Thrust. The Gangdese Thrust is not shown due to the uncertainty of its location.

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Further breakthroughs in understanding the tectonics of the Ti-betan Plateau arose from detailed investigation of high- andultrahigh-pressure metamorphic rocks exposed along the Indus-Yarlung Suture Zone. These are well documented at the classicKaghan Valley and Tso Morari localities in northwestern India,forming the basis for our understanding of continental subductionand high-pressure exhumation during inter-continental collision(e.g. Guillot et al., 2008; Beaumont et al., 2009). Until recently,high-pressure (HP) metamorphic rocks related to India-Asia colli-sion had not been documented in Chinese Tibet (Xizang), possiblydue to difficulty of access. Recent discovery of a blueschist blockin the Yarlung Suture Zone mélange near Sangsang, Tibet (Dinget al., 2005) and high-pressure metasedimentary rocks in theLopu Range (Laskowski et al., 2016) indicate that HP metamor-phism is not limited to thewestern portion of the Indus-Yarlung Su-ture Zone (Fig. 1). With further field exploration and detailedmetamorphic petrology analyses, more evidence for HP metamor-phism is likely to be discovered in Chinese Tibet.

3. Research motivation

Field structural geology remains a critical tool along the Indus-Yarlung suture zone, where large areas have only been mapped at1:500,000 scale. Detailed geologic mapping is critical to under-stand the mechanisms that drove uplift of the Suture Zone frombelow sea level to modern elevations of more than 3500 m. Thenorth-dipping Gangdese Thrust (Yin et al., 1994), thought to carrymagmatic arc rocks of the southern Asian margin over the YarlungSuture Zone assemblages, has been interpreted as a crustal-scalestructure that drove uplift of the Plateau interior (Harrison et al.,1992). However, other workers question the existence of this

structure (e.g. Aitchison et al., 2003), creating a discrepancy inthe literature that can possibly be resolved through field structuralgeology. Furthermore, the timing and possible branching relation-ships between the Gangdese Thrust and splays of the south-dipping Great Counter Thrust system (Fig. 1) are not known. Thissummer, I targeted an exposure of the Indus-Yarlung Suture Zoneeast of Tibet’s second largest city, Xigaze (Shigatse, Rikaze; Fig. 1).This region provides a relatively deep exposure of the SutureZone geology as a result of feedbacks between Yarlung River inci-sion and uplift along a rift flank.

4. Modern conveniences and remaining logistical challenges

The process of conducting field work in Tibet began with theinitiation of work permit applications in October, 2016, enabledby collaborationwith Prof. Lin Ding and Prof. Fulong Cai at the Insti-tute of Tibetan Plateau Research (ITPR) in Beijing, China. In June,2017, I received word from Prof. Fulong Cai that the permits werelikely to be approved in early July, setting the field season in mo-tion. With formal invitation letter in hand, I drove to the ChineseConsulate in Los Angeles with my field supplies to obtain an entryvisa. On July 2, having received final confirmation that the Tibet en-try and work permits would arrive within days, I booked a flight toBeijing only 13 h before it was scheduled to depart.

Upon my arrival in Beijing, I met another colleague from ITPR,Dr. Houqi Wang, to travel back to Beijing Capital Airport. His assis-tance was critical, as Tibet permits are only allowed to be in thepossession of Chinese nationals. Upon arrival at the airport, wenoticed that many flights were delayed or canceled due to rainand fog. We spent most of the day waiting for an update on ourflight, which was eventually canceled. The next few hours were a

Figure 2. (A) The author collecting structural data at an outcrop in eastern XigazeCounty. Photo by He Songlin. (B) The field team (Songlin He, Yaofei Chen, and theauthor from left to right) atop a ridge near the town of Dazhuka, overlooking the Yar-lung river. View is to the west. (C) Outcrop photo of amphibolite block-in-matrixtexture in sedimentary-matrix mélange.

A.K. Laskowski et al. / Geoscience Frontiers 9 (2018) 591e594 593

whirlwind of locating formerly checked baggage, rebooking flights,and attempting book a hotel that accepted foreigners. Finally,around 6 PM, we secured passage on a plane set to depart thenext day. The next challenge was to leave the airport, as the queuefor taxis was massive due to the large number of flight cancella-tions. An overheating taxi that caught fire in explosive fashion,sending the crowd running for cover, provided a fitting crescendo.As I arrived at my hotel next to the ruins of the old Beijing CityWall,I concluded that travel to Tibet was still challenging, even with themodern conveniences of air travel and smartphones.

The second attempt to fly to Tibet was successful, despite anadditional 3 hrs delay in Beijing. We landed at the Lhasa GonggarInternational Airport at night in a thunderstorm, providing dra-matic illumination of the high peaks in the Gangdese Mountains(Fig. 1). At the airport we met our driver, Tuding, who transportedus to the ITPR facility in Lhasa. Unfortunately, the delay in Beijingled to a three-day permitting delay in Lhasa, as the regional govern-ment offices were closed for the weekend. The next few days werespent planning fieldwork logistics, shopping for food and field sup-plies, and dining Sichuan style. I spent a festive evening at the homeof a Tibetan woman named Zhouma, who provides drivers and ve-hicles for geologists working in southern Tibet. She served tradi-tional yak butter tea and yak jerky at her home, then coordinateddinner at a nearby restaurant with a large group of graduate stu-dents from ITPR. I circumvented some communications challengesby “friending” the group onWeChat, a very popular free messagingapp with built-in Chinese-English translation.

At the Institute in Lhasa, I met Yaofei Chen and Songlin He, grad-uate students in Prof. Lin Ding’s research group, who would helpme with fieldwork for the duration of my time in Tibet. Theywere each beginning newprojects, Yaofei Chen is a PhD and SonglinHe is a M.Sc., and were eager to help out despite having alreadyspent a month in Tibet assisting others. In addition, we met ourdriver, Longdou, whom I had previously worked with during fieldwork in 2014. On Monday morning, we set out for Xigaze Countyon Highway 318, sometimes nicknamed the “Friendship Highway”as it connects Lhasa with Kathmandu, Nepal. After proceedingsouth along the Lhasa River, our path turned west and proceededthrough a spectacular gorge cut by the Yarlung River as it crossesthe rift-flank uplifts of the Ringbung Graben. Once on the otherside, we entered the field area (Fig. 1) at the mouth of the gorgealong the Yarlung Suture Zone in eastern Xigaze County. The localpolice prohibited us from camping near the field area, so we droveanother 80 kmwest to Shigatse and reserved rooms in the ShigatseYak Hotel.

5. Field work

Once settled in Xigaze, we established a daily routine that madefor enjoyable and efficient field work. In the mornings, we ateChinese-style breakfast in the lobby of the Yak Hotel. On a few oc-casions, Longdou shared some tsampa (roasted barley flour mixedwith water) and Tibetan yogurt, making for a more traditionalexperience. Eachmorningwe drove east on the 318 back to the fieldarea, which usually took about an hour. As field work progressedwe worked through a list of targeted traverses I had mapped inGoogle Earth based on satellite imagery and existing geologicmaps. After a few days in the field, I realized that the steep moun-tainsides appeared much easier to climb in Google Earth, and mosttraverse targets had to be split into segments. Our group of threeshared the field work tasks; I was in charge of the field notes, struc-tural measurements, and mapping (Fig. 2A) while Yaofei Chen andSonglin He (Fig. 2B) collected samples, carried samples, and oper-ated the handheld GPS. In all, we spent two weeks in the field,including two rest days in Xigaze.

The most exciting geology was exposed immediately south ofthe Yarlung river, east of the town of Dazhuka (Fig. 1). Mapping inthis area revealed multiple splays of the Great Counter thrust sys-tem, a structurally-deeper, north-dipping mylonitic shear zonethat cut magmatic arc rocks (likely equivalent to the Gangdesethrust), and serpentinite- and sedimentary-matrix mélanges. Alarge, garnet-bearing amphibolite block in the sedimentary-matrix mélange became a highlight of our field work, as its para-genesis and dateable minerals (e.g. zircon, apatite, garnet) possiblyrecord a robust pressure-time-temperature history. The margins ofthe amphibolite block displayed an outcrop-scale block-in-matrixtexture, potentially providing a useful analog for typical deforma-tion in an accretionary wedge setting (Fig. 2C).

6. Conclusions

The frontier status of the Yarlung Suture Zone provides an op-portunity for geologists to discover new information about the tec-tonic processes that created the Tibetan Plateau. The field worksummarized in this paper is a small contribution towards construc-tion of a comprehensive tectonic model for inter-continental

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collision, which creates some of Earth’s largest mountain belts.Modern conveniences make accessing southern Tibet easier thanever before, but there are still logistical challenges that must beovercome.

Acknowledgements

This research was funded by Montana State University and byresearch grants to Prof. Lin Ding, ITPCAS, including National KeyResearch and Development Plan (Grant No. 2016YFC0600303)and the National Natural Science Foundation of China (Grant No.41490615).

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Andrew K. Laskowski*Department of Earth Sciences, Montana State University, Bozeman,

MT, USA

Lin Ding, Fu-Long Cai, Yao-Fei Chen, Song-Lin HeKey Laboratory of Continental Collision and Plateau Uplift, Institute ofTibetan Plateau Research, and Center for Excellence in Tibetan PlateauEarth Sciences, Chinese Academy of Sciences, Beijing 100101, China

*Corresponding author.E-mail address: andrew.laskowski@montana.edu (A.K. Laskowski)