GEOLOGICAL SIGNIFICANCE OF THE UPPER MEKONG RIVER
AND ITS TRIBUTARIES

By the mid 1970s, the theory that the Himalayas were produced by a collision between India and Asia was becoming widely accepted (Figure 3), but it raised major questions regarding the displaced mass of Asian crust. Since about 50 mya, it appeared that India had penetrated at least 2000 km into Asia, but it was not clear that the mass of the Himalayas and the thickened crust beneath the Tibetan Plateau were sufficient to account for the entire mass of displaced Asian crust. Using satellite images and seismic analysis, Molnar and Tapponnier (1975) recognized major east-trending, left-lateral strike-slip faults emanating from Tibet. These faults sliced the areas east and southeast of Tibet, implying that crustal mass north of the Himalayas was shifting eastward and southeastward as India moved north (Figure 4). Molnar and Tapponnier hypothesized that the Yunnan Plateau and the Shan-Thai Plateau are now much further southeast than they were before the collision (Figure 5).  

We decided that the Mekong River Canyon in western Yunnan was the best region for examining faults to determine whether large blocks of southeast Asian crust were being extruded southward towards Indochina. The river crosses several faults in a canyon two miles deep in the stretch where it changes direction from south to southeast (Figure 6). We hoped to investigate the nature of these faults to determine their relationship to the development of the Himalayas and the uplift of the Tibetan Plateau.

To achieve this objective, our plan was to determine the sense of shear in mylonites, to sample shear zones for radiometric dating, and to sample overlying Triassic red beds to measure paleomagnetic latitude and orientation. Mylonites are fault rocks, which are cohesive, characterized by a well-developed schistosity resulting from tectonic reduction of grain size, and commonly contain rounded porphyroclasts and lithic fragments similar in composition to the matrix minerals. Fine scale layering and an associated mineral or stretching lineation are commonly present. Brittle deformation of some minerals may be present, but deformation is commonly by crystal plasticity (Brodie et al. 2004).

A related objective was to take paleontological samples in hopes of justifying further research on patterns of environmental change as the rising Himalayas modified the climate of this area. Will was one of the oarsmen and the only paleontologist on the team.

In southwestern Yunnan, the Mekong River weaves back and forth across the contact between metamorphic rock units and Mesozoic sedimentary strata (Figure 7). According to Chinese geologists, the metamorphic rocks are of amphibolite grade, and therefore temperatures in them have exceeded 450°C. More importantly, the metamorphic rocks are strongly sheared, with a near vertical northwest-trending foliation and subhorizontal lineation characteristic of rocks that have experienced high shear. Neither the sense of shear nor their age was known. On Chinese maps, the sedimentary rocks were shown as Jurassic and Cretaceous shallow marine deposits in fault contact with the underlying metamorphic rocks. Determining the nature of this contact was an important part of our research proposal.  

In summary, we planned to examine in detail the deformation and rotation of rocks exposed along the Mekong River in western Yunnan to characterize the age, style, intensity, distribution, and sense of crustal shearing induced by the collision and penetration of India into Asia. We hoped to constrain the extent and the timing of extrusion of parts of Indochina from an initially more northwesterly position and to place bounds on the amount and distribution of shearing in western Yunnan due to India sliding northward past Yunnan. We believed that a detailed study of one key area was a better approach than a superficial examination of a large area.

The stretch of Mekong that we planned to run appeared to offer a wide range of possible approaches to studying this deformation. Knowledge gained from this area would be fundamental to understanding areas of more complex deformation in northern Yunnan, western Sichuan, Tibet, and Qinghai. We hoped to expand our collaborative research program to these regions, concentrating on areas where our collective expertise could have been used to maximize the results of our research. Although Will’s scientific background and interest were generally limited to the paleontological aspects of this expedition, his role as an experienced whitewater river guide who could read, write, and speak Mandarin was critical to the success of the expedition.