The Southern Tibetan Plateau


Sangxung & Gulu Graben

Gulu Graben & Nyainqenttanglha Mountains

The snow-covered Nyainqenttanglha Mountains appear on the left side of this Space Shuttle photograph with the Nam Tso lake to the northwest. The Gulu Graben forms a dramatic NE-SW trending valley to the east of the range. It connects with the smaller N-S trending Sangxung Graben at Horru. The Yangbajain geothermal field, which provides about 50% of Lhasa's power needs, is located at the southern end of the graben. The valley of the Lhasa River crosses the lower right corner; the city is just off the lower edge of the photograph.


Faults Bounding the Sangxung Graben - J.H. Wittke

 

South of Sangxung, we enter the northern end of the Gulu-Yadong graben. Dramatic fault scarps, about 1 m high, line the west side of the Gulu valley, steep ruptures in the ground that reveal the red soil and run like long ruddy ribbons across the slope, which formed when the valley floor was lowered relative to the surrounding mountains. In places where movement was distributed over a series of small faults rather than a single large one, the scarps break into a series of smaller steps, producing a braided pattern on the hillsides. The lack of weathering and paucity of vegetation indicate that these surface breaks are very young; older scarps would have more subdued slopes and be overgrown.

The southern part of the Tibetan Plateau is broken by an extensive set of grabens, forming linear valleys that are aligned roughly N-S (averaging 6o E of N). These structures mark a shift in stress for this portion of the crust, formed as the Tibetan Plateau expanded eastward. Earthquake data reveal that E-W extension presently prevails throughout the Plateau with blocks of crust moving vertically relative to one another. In essence, the Tibetan Plateau is collapsing and spreading gravitationally, having attaining its maximum sustainable elevation in the Early Pliocene.

Continental crust can be thickened only so much for a given convergence rate and temperature distribution within it. Continental crust, like a piece of taffy, will gradually spread out under its own weight. If we confine the taffy in a paper wrapper, or squeeze it gently, its collapse will be slowed or stopped. Comparably, the collision of India and Eurasia helps support the height of the Tibetan Plateau.The time when the collapse began may also mark the time when the Plateau first reached its maximum height. Thermochronological studies of the Nyainqêntanglha range on the west side of the Gulu graben by Mark Harrison (UCLA) and his colleagues have used to constrain the initiation of extension to 8 ± 1 Ma. If the Gulu graben is representative of southern Tibetan grabens in general, this age of maximum elevation applies for all of the Tibetan Plateau.

Chorten at Horru - J.H. Wittke

Horru

The road climbs steadily and turns southwestward at the village of Horru. Here a line of eight weathered chörten marks the top of Kyogche La at 4900 m (16,070'). They stand like battered soldiers in a row perpendicular to the road. Each is capped by a spire composed of thirteen dirty yellow disks that decrease in size smoothly toward the inverted cones and sun-moon talismans that form the top. The Tibetans have strung strings of prayer flags around the chörten. Two conical incense burners stand at the end of the row nearest the road. The beehive-shaped burners are made of flat rocks and sit on square dry-stone bases. Piles of stones flank each incense oven, supporting clusters of thin sticks that sag under their devotional loads of prayer flags.

Below the summit, the road descends into the Dam Qu valley, which is also part of the Gulu-Yadong graben system. The Dam Qu valley is unusual because of its northeast-southwest orientation. Although it is also the result of E-W extension, pre-existing weaknesses in the crust controlled its orientation. Geologists term this effect "reactivation," because fractures and faults formed during older tectonic events are reused during younger deformations. It is extremely common for an earlier structural "grain" to control subsequent geological activity, whether it be later faulting or the location where molten rock emerges from the crust. As is the case with people, much of how the Earth is today has been inherited. It is important in geology to distinguish between geometry (surface orientation) and kinematics (movement direction). The extension directions indicated for the graben are oblique to its orientation, being oriented essentially E-W (averaging 7oS of E).

Mt. Nyainqenttanglha - J.H. Wittke

Nyainqên Tanglha Range

The road along the Dam Qu passes small undistinguished villages. Their stone walls are capped by piles of fuel, consisting of sticks and yak dung patties that have been dried in the sun. To the west rise the Nyainqên Tanglha Mountains, capped with snow. Mount Nyainqêntanglha, at 7160 m (23,490'), the highest peak in the range, is one of Tibet's sacred peaks and is said to resemble a glistening white knight astride a charger. Unfortunately, we can only see the lower slopes; a wreath of gray clouds enfolds the summit.

The Nyainqên Tanglha Mountains are formed of granite intruded into high-grade schist and gneiss, rocks that originally formed about 15 km below the surface at temperatures in excess of 600oC. There is some evidence that the granite formed about 50 Ma, making it contemporaneous with the initial collision of the Indian subcontinent with Eurasia, however the dating results are not particularly trustworthy. The gentle eastern slope of the range preserves a shear zone that predates the normal faulting, which formed the valley. Deformation is distributed across a wide band of rock in this ductile version of a fault, rather than being concentrated along a single plane of breakage. The resulting mylonitic rock textures are quite dramatic, reflecting immense amounts of shear at temperatures greater than 300oC at probably more than 10 km depth.

The UCLA group has demonstrated that the Nyainqêntanglha rocks underwent rapid cooling 8-4 Ma as the west side of the zone was rapidly uplifted. This uplift, driven by isostasy, caused the shear zone to rotate from its original inclination of about 60o, to its present dip of about 25o. The cooling caused by uplift and rotation had the effect of "locking up" the shear zone by moving it into a mechanically unfavorable angle, preventing further motion. As extension continued, new younger faults formed at steep angles. This progression of fault formation, rotation, inactivation, and the initiation of new steeper faults is identical to that observed in metamorphic core complexes of western North America, which occur all along the Cordilleran mountain range. The discovery of similar structures in Tibet, associated with gravitational collapse of the Plateau, suggests that the elevated western U.S. was once significantly higher and may also have failed gravitationally.

Yangbajain

Yangbajiang is host to a developed geothermal field, but we arrive too late to visit the plant located just west of town. Here the road makes a ninety-degree turn and follows the Doilung Qu towards Lhasa. The river pours all of its considerable flow through a narrow canyon in its race to join the Lhasa River and ultimately the Yarlong-Tsangpo (Indus) River. A series of spectacular rapids fill the canyon, contained between granite walls. In places, the river gushes between cliffs less than a 100 m apart, a churning torrent of yellow-brown water.

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