Tectonics and geology

The activities of the Bulgarian-Chinese Speleological Expedition (2011) were concentrated in Western Yunnan, in the southern parts of the Gaoligong Shan Range, in regions situated between Nujiang (Salween) River, to the East, and Myanmar border, to the West.

Western Yunnan is situated in the southeastern part of the Himalayan Mountain Belt close to the Myanmar border (Fig. 1). There are some narrow mountain chains, elongated in N-S direction, which belonged to the SE branch of the Eastern Tethyan Orogene and, therefore, beared significance of the Tethyan Belt in SE Asia. The Himalaya was formed during the collision between the Indian and Eurasian Plates. This collsion ensures the transition from a compressional regime in the Himalayas to a right-lateral shear in Myanmar-West China Zone, where a couple of joined smaller continental blocs occured (Fig. 2). The process is started in the Late Mesozoic and it continues during the Cenozoic till now. The present day deformation occurs throughout the Tibetan Plateau interior by ESE-WNW extension and slightly slower NNE-SSW shortening, which there accommodates much of India’s penetration into Eurasia (Figs. 3, 4). Relative to Eurasia, material within the plateau interior moves roughly eastward with speeds that increase toward the east, and then flows southward around the eastern end of the Himalaya (Zhang, et al., 2004). The Gaoligong Shan is the westernmost part of this mountain range. It is formed mainly in the eastern part of the Tenchong Continental Block, which is bounded to the East by the Nujiang Fault Line (Figs. 5, 6). However, the western boundary of the block is ambiguous.

The Gaoligong Shan Range is about 3000 meters high. It limits the Baoshan area and the Tengchong/Ruili area. The core of the range is composed of low to high grade metamorphic rocks and foliated granites. They are intensely deformed and affected by a subvertical foliation, dipping toward in both, east and west directions, as well as isoclinal ductile folding. Sheet folds and mineral lineation parallel to the range trend indicate severe stretching (Fig. 7). Ductile shear-sense criteria show a right- lateral motion. The mylonitization is dated between 12 and 20 Ma (Zhong et al., 1991; Wang and Burchfiel, 1997). East of the range, Nujiang Detachment Fault separates the metamorphic rocks from the folded Paleozoic sediments of the Baoshan Block. Southernmost ophiolitic rocks are pinched between the metamorphic rocks and the Paleozoic sediments. In the western part of the range, foliated granites are exposed. Neogene deposits overlie them. To the west, the basement, composed of flat foliated granites and metamorphic rocks, is intruded by recent volcanic rocks of the Tengchong Rift (Socqueta and Pubelliera, 2005).

Southward the mylonitic belt disappears near Longling Town. A large batholith intrudes the metamorphic range, which seems to curve to the southwest. An earthquake swarm occurred in 1976 (MZ5.29–7.1), which have been interpreted to be related to left-lateral movement along NE trending faults (Holt et al., 1991), indicate that this segment of the fault zone is still active.

In the present position, the Tengchong Block and its eastern neighbor - the Baoshan Block are separated by the Nujiang Fault (Fig. 6). The basement of two blocks is largely covered by medium-grade to low-grade metamorphic Paleozoic to Early Mesozoic sedimentary rocks. The strata were strongly folded. These rocks are intruded by numerous granitoids mainly Mesozoic to Cenozoic in age. Active faults are distributed allover the area.

Sedimentary rocks of the Tengchong Block occurred in a roughly N-S zone in the center of the block. They are more or less metamorphosed (Fig. 7). The Permo-Carboniferous sequence constitutes most of them. It is composed of the clastic Menghong Group and the overlying carbonate Dadongchang (Yanzipo in southernmost Tengchong) Formation (Fig. 8).

The Menghong Group starts with 600-700 m of shallow marine deposits: monotonous mudstones, siltstones and sandstones. The contact with older sequences is either faulted or covered by Cenosoic deposits. In the middle part of the group, there is a succession of 700-900 m of coarse sediments, comprising dominantly diamictites and pebbly mudstones. Slumps and dewatering structures, sand pillows, wavy bedding in fine intercalations are seen in this portion. Large clasts of the diamictites and pebbly mudstones are composed of a variety of rock types. The most common of them are quartz, granite, metamorphic rocks, cherts, sandstone, siltstone and mudstone. The upper part of the Menghong Group is composed of 200-400 m of dark to black shales and siltstones with some irregular argillaceous and limestone beds. This part of the section contains bryozoans, crinoids, brachiopods and bivalves - the typical fossil association of Permo-Carboniferous periglacial marine deposits (Xiaochi, 2002).

Limestones, dolomitic limestones and marbles of the Dadongchang Formation, or of the similar Yanzipo Formation in southernmost parts of Tengchong Block overlie the Menghong Group. The thickness of carbonate strata is about 400 m. The lower part consists basically of bioclastic limestones, and the middle and upper parts - more dolomitic limestones and some chert nodules and bands. The fossil fauna guides to suppose the age of carbonates to be Late Carboniferous to Early Permian, or more precisely Asseian (?) to Wuchiapingian (Geological Survey of Yunnan, 1986). The top part of the Permian is unknown. In the northern part of Tengchong Block, on the western flank of the Gaoligong Shan Range, there are some dolomitic and argillaceous limestones that have been mapped as the Middle Triassic. Probably these carbonates also contain Lower Triassic strata (Xiaochi, 2002).

Upper Paleozoic glaciomarine deposits combined with the presence of cold-water faunas and Glossopteris indicate a Gondwana provenance of the Tengchong and Baoshan Blocks in Western Yunnan. These two terrains separated from Gondwana in the late Early Permian. Docking commenced in the Late Triassic concomitant with the closure of the Changning-Menglian Belt (Figs. 5, 6). Lateral displacements in the course of the Himalayan Orogeny moved the Tengchong Block to north, bringing it into juxtaposition with the western margin of the Baoshan Block. This tectonic contact is now the Nujiang Line (Wopfner, 1996).

Fig. 1. Geographical settings. The observed area is shown with a red rectangle.

Fig. 2. Continental blocks and orogenic belts that make up Southwest China (modified from Jin, 1998). The observed area is shown with a darken rectangle.

Fig. 3. Sketch showing the mechanism of tectonic deformation in SE Asia (modified from Ching et al., 2007). The observed area is shown with a darken rectangle.

Fig. 4. A present day global positioning system (GPS) velocities (mm/yr) in and around Tibetan Plateau with respect to stable Eurasia, plotted on shaded relief map (modified from Zhang et al., 2004). The observed area is shown with a red rectangle.

Fig. 5. Tectonic map of the Western Yunnan area (Akciz et al., 2008). The observed area is shown with a darken rectangle.

Fig. 6. Composite map of the Tenchong and Baoshan blocks (Xiaochi, 2002). The observed area is shown with a darken rectangle.

Fig. 7. E–W cross-section of the Gaoligong Shan (Socquet and Pubellier, 2005).

Fig. 8. Composite stratigraphic column of the Permo-Carboniferous in the Tengchong Block (Xiaochi, 2002).

Cave prospecting results in the light of the local geological and tectonic settings