94˚15 ’ 94˚30’ Yigrong Tsangpo94˚45’ 95˚00’ 95˚15’ 95˚30’ 95˚45’ 96˚00’ 30˚15 ’ Jiali #5 – Sediment Fluxes and Geomorphology Geologic Hazards Associated With H11C - 0644 Around the Yarlung-Tsangpo canyon, slopes are very steep, 5 mm/yr or more, with the possibility that local bedrock inci- Fault Bode Tsangpo relief is extreme (Figure 7, bottom), and erosion is landslide- sion rates are much higher. There is enormous stream power Po Tsangpo Po Tsangpo 30˚00’ dominated. Erosion rates integrated over timescales ranging at the knickzone (Figure 7, top), and the sediment $uxes de- a Proposed Dam on the Yarlung- from millions of years to the present, using techniques includ- veloped in this region are enormous (Figure 8). 1580m ing petrology, thermochronology, cosmogenic isotopes, and Gyala Peri 7151m 2500m Dam? detrital dating all converge on sustained erosion rates of some Tsangpo River in SE Tibet Zone 29˚45’ Namche Barwa 7782m Nyingchi R. 800m 6000 40 Peter K. Zeitler ([email protected]), Anne S. Meltzer ([email protected]) 2900m (first rapids) Lhasa block sedimentary B. rocks Figure 7. Plots comparing cooling ages, stream power, Biotite 40Ar/39Ar Mean Annual 35 29˚30’ Lhasa block meta- 5000 River Power (watts/m2) Age (Ma) sedimentary rocks & granite relief, and the Tsangpo’s long pro!le in and near the ) Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 US ) (Lhasa block basement 30 knickzone. From Finnegan et al. (in press). 4000 Bernard Hallet ([email protected]) Younger granites 25 Gangdese plutons 3000 20 Yarlung Tsangpo ductile shear/mylonite zone Ophiolitic rocks of Tsangpo Suture Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195 USA 29˚15’ Yarlung Tsangpo Zircon (U-Th)/He 15 Ductile shear zone Tethyan Himalayan 2000 metasediments Age (Ma) normal sense 10 (Ma) Age Cooling Planar mylonites Migmatites and mylonitic William S.F. Kidd ([email protected]) thrust sense gneisses; thrust sense (W/mRiver Power 1000 shear zone boundary 5 Sense of ductile shear: 8 ? Greater Himalayan(?) A A' thrust; normal metasediment Parlung River 0 0 Department of Earth and Space Sciences, University at Albany, Albany, NY 12222 USA tribs., Tibet ? sinistral; dextral Gneisses (Indian? basement) Tsangpo River ? 50 km n = 145 29˚00?’ pale area - garnet-rich felsic Nyang R., Tibet A C. ? WSFK 20060811 Shear zone boundary and mafic gneisses Incised Terraces ? n = 50 3000 5000 Peter O. Koons ([email protected]) 6 Braided Reach 4500 2500 Figure 4. Geological map of SE Tibet near the Big-Bend knickzone, showing Namche 4000 Department of Earth Sciences, University of Maine, Bryand Global Sciences Center, Orono, ME 04669 USA 2000 Barwa-Gyala antiform. Active faults drawn in black. Approximate location of the 4 Relief (m) 3500 1500 dam and diversion tunnel proposed in some schemes shown by dashed red oval. 3000 Probability 1000 Relief (m) Local River Elevation (m) River Elevation 2500 2 500 2000 Siang River A' Pasighat, India 0 1500 #1 – Background #3 – Active Tectonics: Structure and Antiform Evolution n = 101 0 100 200 300 400 500 600 700 0 River Distance (km) For over a decade anecdotes and media reports have been circulat- as an equally pressing need to provide large amounts of cleaner The Big-Bend knickzone is developed across an active an- the knickzone is likely pinned at the growing antiform 0.1 1 10 100 ing about a proposed dam on the Yarlung- Tsangpo River in SE energy, we feel it timely to assess the impact of a Tsangpo dam. tiformal metamorphic massif that exposes Indian-plate and that in fact the two features are part of a coupled Zircon Fission-Track Age (Ma) Tibet. The proposed site is in the deep canyon of the Yarlung- basement (Figure 3) as a result of a recent and probably sytem in which erosion localizes and enhances deforma- Tsangpo, where the river leaves the Tibetan Plateau, falling ~2000 O"setting bene#ts that a dam would o"er in the form of improved ongoing tectono-metamorphic episode . Granite dikes tion and local rock uplift. Biotite %&Ar/'(Ar cooling ages Figure 8. Fission-track data from detrital zircons obtained meters across an immense knickzone developed along an irregular water supply to northern China, signi#cant water-$ow and from sands sampled upstream and downstream of the and migmatites give U-Pb zircon ages from 1 to 10 Ma, break sharply across what appear to steep massif-bound- Big-Bend knickzone. Compare the distribution of ages U-shaped reach ~100 km in length (Figures 1 to 3). sediment-$ux impacts would be felt downstream in the Brahmapu- and metamorphic geochronology and P-T work shows ing faults (Figure 5a), consistent with a continuum of ac- tra system in northeastern India and Bangladesh, and hazards asso- observed downstream at Pasighat, India with those ob- that the massif has experienced ~6 mm/yr of exhumation tivity that continues to the present. tained from the Tsangpo upstream of the knickzone: The fundamental purpose of the dam would be generation of po- ciated with a dam located in such a geologically active area would during at least the past 3-4 m.y. Our studies suggest that some 50% of the downstream zircons are very young tentially as much as ~40,000 MW of hydropower to be used in di- be substantial. Further, the dam and reservoir would likely ruin the (less than 2 Ma; mode at 0.6 Ma). verting a portion of the impounded river to water-starved regions pristine, ecologically and ethnographically diverse Pemako region of northern China (for comparison, the Three Gorges project is rated around the Yarlung-Tsangpo canyon, an area of great signi#cance to The only possible source is the Namche Barwa massif (see at slightly more than 20,000 MW). If pursued, the Tsangpo dam Tibetan Buddhists. Figure 5a. Biotite %&Ar/'(Ar cooling ages. This dating Figure 5b. Zircon U-Th/He cooling ages. This dating system Figure 5b). Coupled with estimates of total sediment "ux would be part of the broader “Western Route” of the nan shui bei system has a closure temperature of about 350˚C. has a closure temperature of about 200˚C. measured at Pasighat, these data suggest a modern diao water-diversion scheme (“transferring water from the south to We have been examining the geodynamic evolution of eastern mean erosion rate of 13 ± 6 mm/yr in the area surround- the north”). We have no concrete knowledge that a Tsangpo project Tibet and now have considerable geophysical and geological data 30˚30' Cooling Ages ing the knickzone, supplying some 100 Mt/yr of sediment is in active planning, and o!cials have downplayed stories about that bear on the siting of a dam at this location. Our conclusion is (Ma) to the Tsangpo. From Stewart et al. (in review). such reports (Biron and Dodin, 2007). However, given the persistent that despite the allure of its vast hydropower potential, the Big- >50.0 media reports, the pressing water-resources issues in China as well Bend knickzone is a dangerously unsuitable place to site a dam. 50.0 20.0 30˚00' 15.0 10.0 90 98 32 5.0 92 94 96 Y i d u Bangong SutureQiangtang Southern Tibetan A r c 2.5 STDS Te r r a n e Figure 9. Summary of data relevant to dam siting. Detachment System Lhasa Block 1.0 GHS Greater Himalayan Sequence Jinsha Suture LHS Lesser Himalayan Sequence Jiali Fault 29˚30' 1600 m ? ? #6 – Implications for Dam Siting and Hazards 30 Elevation (m) GP Nyang R. Lhasa NB Bangong Suture 7000 Tsangpo R. Gangdese Thrust Our data indicate that any dam placed within the would lead to rapid siltation. Further, this impoundment 6000 Indus Tsangpo Suture Yarlung-Tsangpo canyon would be at high risk, with the of the Yarlung-Tsangpo would greatly starve the sedi- ? 5000 Renbu Zedong Thrust Tethyan dam and diversion being prone to failure due to pro- ment $ux downstream into the Brahmaputra and ulti- 4000 Himalaya STDS 29˚00' STDS 28 nounced seismic hazards and focused deformation. As it mately the Bay of Bengal systems. As such, the dam, 2750 m 2200 m Main Central Thrust 3000 30˚30' #lls, water pressure behind the dam could help trigger which would be built at the cost of an ecologically pre- GHS 2000 LHS shallow earthquakes and landslides, and the dam would cious region, would likely not succeed in its purpose 1000 m 1000 LHS be di!cult to maintain given the high frequency of land- while causing signi#cant harm locallly and downstream. Main Boundary Thrust 0 Brahmaputra R. sliding and extreme local bedrock exhumation rates that Depth (km) Figure 2 (above). Cartoon showing main geologic elements 30˚00' 0 around the knickzone and Namche Barwa - Gyala Peri massif (dashed box). 10 GP GP 2900 m 20 850 m Acknowledgements 90˚ 92˚ 94˚ 96˚ 98˚ NB 30 NB Thanks to all the members of the indentor corners project who helped collect the data we are presenting here. 29˚30' Figure 1 (above). Google Earth image showing location of proposed dam at Big- ? ? 40 This work was funded by the Continental Dynamics Program at the U.S. National Science Foundation, and was dam 30˚ Bend knickzone. Wedge shows path for diversion tunnels into downstream dam. 30˚ also supported by IRIS-PASSCAL. Approximate extent of 250 m and 500 m impoundments shown in blue. site 50 28˚ 28˚ References 29˚00' #2 – Geodynamic Context 94˚00' 94˚30' 95˚00' 95˚30' 96˚00' 94˚00' 94˚30' 95˚00' 95˚30' 96˚00' Biron, C.L., and Dodin, Th., 2007.