Geologic Hazards Associated With a Proposed Dam on the Yarlung-Tsangpo River in SE TibetPeter K. Zeitler (peter.zeitler@lehigh.edu), Anne S. Meltzer (ameltzer@lehigh.edu)Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 USBernard Hallet (hallet@u.washington.edu)Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195 USAWilliam S.F. Kidd (wkidd@atmos.albany.edu)Department of Earth and Space Sciences, University at Albany, Albany, NY 12222 USAPeter O. Koons (peter.koons@maine.edu)Department of Earth Sciences, University of Maine, Bryand Global Sciences Center, Orono, ME 04669 USA

Thanks to all the members of the indentor corners project who helped collect the data we are summarizing here. This work was funded by the Continental Dynamics Program at the U.S. National Science Foundation, and was also supported by IRIS-PASSCAL.

Acknowledgements

Figure 4. Geological map of SE Tibet near the Big-Bend knickzone, showing Namche Barwa-Gyala Peri antiform. Observed active faults drawn in black (others may also be active, to SE of massif along lower Tsangpo). Approximate location of the dam and diversion tunnel proposed in some schemes shown by dashed red oval.

Figure 7. Plots comparing cooling ages, stream power, relief (blue circles), and the Tsangpo’s long pro�le in and near the knickzone. From Finnegan et al. (in press).

Figure 9. Summary of data relevant to dam siting: active faults, seis-micity (orange dots), and biotite and zircon cooling ages less than 2.5 Ma (red and light-colored circles; a proxy for high erosion rates).

Figure 8. Fission-track data from detrital zircons obtained from sands sampled upstream and downstream of the Big-Bend knickzone. Compare the distribution of ages observed downstream at Pasighat, India with those ob-tained from the Tsangpo upstream of the knickzone: some 50% of the downstream zircons are very young (less than 2 Ma; mode at 0.6 Ma).

The only possible source is the Namche Barwa massif (see Figure 5b). Coupled with estimates of total sediment �ux measured at Pasighat, these data suggest a modern mean erosion rate of 13 ± 6 mm/yr in the area surround-ing the knickzone, supplying some 100 Mt/yr of sediment to the Tsangpo. From Stewart et al. (in review).

For over a decade anecdotes and media reports have been circulat-ing about a proposed dam on the Yarlung- Tsangpo River in south-eastern Tibet. The proposed site is in the deep canyon of the Yarlung-Tsangpo, where the river leaves the Tibetan Plateau, falling ~2000 meters across an immense knickzone that has developed on an irregular U-shaped reach ~100 km in length (Figures 1 to 3).

The fundamental purpose of the dam would be generation of po-tentially as much as ~40,000 MW of hydropower to be used in di-verting a portion of the impounded river to water-starved regions of northern China (for comparison, the Three Gorges project is rated at slightly more than 20,000 MW). If pursued, the Tsangpo dam would be part of the broader “Western Route” of the nan shui bei diao water-diversion scheme (“transferring water from the south to the north”). We have no concrete knowledge that a Tsangpo project is in active planning, and o�cials have downplayed stories about such reports (Biron and Dodin, 2007). However, given the persistent media reports, the pressing water-resources issues in China as well

as an equally pressing need to provide large amounts of clean energy, we feel it timely to assess the impact of a Tsangpo dam.

O�setting bene�ts that a dam would o�er in the form of improved water supply to northern China, signi�cant water-�ow and sediment-�ux impacts would be felt downstream in the Brahmapu-tra system in northeastern India and Bangladesh, and hazards asso-ciated with a dam located in such a geologically active area would be substantial. Further, construction of the dam and �lling any res-ervoir would likely ruin the ecologically and ethnographically di-verse and pristine Pemako region around the Yarlung-Tsangpo canyon, an area of great signi�cance to Tibetan Buddhists.

We have been examining the geodynamic evolution of eastern Tibet and we have considerable geophysical and geological data that bear on the siting of a dam at this location. Our conclusion is that despite the allure of its vast hydropower potential, the Big-Bend knickzone is a dangerously unsuitable place to site a dam.

#1 – Background

The Big-Bend knickzone is developed across an active an-tiformal metamorphic massif that exposes Indian-plate basement (Figure 4) as a result of a recent and probably ongoing tectono-metamorphic episode . Current surface exposures of granite dikes and migmatites give U-Pb zircon ages from 1 to 10 Ma, and metamorphic geo-chronology and P-T data show that the massif has experi-enced ~6 mm/yr of exhumation during at least the past

3-4 m.y. Our studies suggest that the knickzone is likely pinned at the growing antiform and that in fact the two features are part of a coupled sytem in which erosion lo-calizes and enhances deformation and local rock uplift. Biotite 40Ar/39Ar cooling ages break sharply across what appear to steep massif-bounding faults (Figure 5a), con-sistent with a continuum of activity that continues to the present.

#3 – Active Tectonics: Structure and Antiform Evolution

Around the Yarlung-Tsangpo canyon, slopes are very steep, relief is extreme (Figure 7, bottom), and erosion is landslide-dominated. Erosion rates integrated over timescales ranging from millions of years to the present, using techniques includ-ing petrology, thermochronology, cosmogenic isotopes, and detrital dating all converge on sustained erosion rates of some

5 mm/yr or more, with the possibility that local bedrock inci-sion rates are much higher. There is enormous stream power at the knickzone (Figure 7, top), and the sediment �uxes de-veloped in this region are enormous (Figure 8).

#5 – Sediment Fluxes and Geomorphology

To augment a regional broadband seismic array deployed for 15 months in 2003 - 2004, we installed a denser array of short-period instruments closer to the Namche Barwa massif. Our results show a spectacular clustering of events below its northernmost boundary (Figures 6a and 6b). These do not represent shallow geomorphic events like landslides or edi�ce

failure, as a number of hypocenter locations extend to consid-erable depths of more than 10 km. These must be related to active deformation beneath the massif coupled to stresses contributed by topography. Figures 6a and 6b show a total of 1477 events (mb 1.0 to 5.6, with a range of �rst motions). The possible dam site is located within the dashed red circle.

#4 – Seismicity

Our data indicate that any dam placed within the Yarlung-Tsangpo canyon would be at high risk, with the dam and diversion being prone to failure due to pro-nounced seismic hazards and focused deformation. As it �lls, water pressure behind the dam could help trigger shallow earthquakes and landslides, and the dam would be di�cult to maintain given the high frequency of land-sliding and extreme local bedrock exhumation rates that

would lead to rapid siltation. Further, this impoundment of the Yarlung-Tsangpo would greatly starve the sedi-ment �ux downstream into the Brahmaputra and ulti-mately the Bay of Bengal systems. As such, the dam, which would be built at the cost of an ecologically pre-cious region, would be unlikely to succeed in its purpose while causing signi�cant harm locally and downstream.

#6 – Implications for Dam Siting and Hazards

The proposed dam site at the great Tsangpo knickzone is lo-cated at the easternmost end of the Himalayan arc, where it terminates in the active Namche Barwa - Gyala Peri base-ment massif. At a more regional scale, GPS results show that steep three-dimensional velocity gradients exist across the region (Figure 3), and in the easternmost Hima-laya near Namche Barwa, >50% of the India – Eurasia plate convergence is accommodated within the high-strain zone that reaches to the southern edge of the proposed site. The 1950 Assam earthquake (M8.6; Figure 3) was one ex-pression of the high local strain rates, and caused consider-able damage within the canyon area.

#2 – Geodynamic ContextBiron, C.L., and Dodin, Th., 2007. Siphons to the north. Himal Southasian, v. 20(9). http://www.himalmag.com/2007/september/tibet_dam_mao_zedong.htm.

Finnegan, N. J., Hallet, B., Montgomery, D. R., Zeitler, P. K., Stone, J. O., Anders, A. M., and Liu, Y., in press. Coupling of rock uplift and river incision in the Namche Barwa-Gyala Peri massif, Tibet. Geological Society of America Bulletin.

Stewart, R. J., Hallet, B., Zeitler, P. K., Malloy, M.A., Allen, C. M., and Trippet, D., in review. Brahmaputra sediment �ux dominated by highly localized rapid erosion from the easternmost Himalaya. Geology, in review.

References

Geodynamics of Indentor CornersMore Information: http://www.ees.lehigh.edu/groups/corners

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Figure 3 (right). DEM showing location of proposed dam and its context in the geography of SE Asia. GPS vectors are also shown (Eurasian reference frame; for scale, highest veloc-ity is about 40 mm/y). Orange vectors are notional: they translate measured Shillong values across Assam, assum-ing block behavior of Indian plate. They illustrate the po-tential for signi�cant strain ac-cumulation at the northeast-ern end of the Himalayan arc.

– locations of 1897 Shillong and 1950 Assam great earth-quakes.

Figure 1 (above). Google Earth image showing location of proposed dam at Big-Bend knickzone. Wedge shows path for diversion tunnels into downstream dam. Approximate extent of 250 m and 500 m impound-ments shown in blue.

Figure 2 (above). Cartoon showing main geologic elements around the knickzone and the Namche Barwa - Gyala Peri massif (dashed box).

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