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

During the Bulgarian-Chinese Speleogical Expedition (2011) were explored caves located in different geological and tectonic circumstances. The major part of them are typical karst phenomena developed in the Lower Permian carbonatic deposits of the Dadongchang Formation. The dolomites and the limestones of the formation are intensively folded. In some areas, they are partially or fully transformed into marbles. Many contemporary and older fracture networks, brittle and ductile faults cut through the strata.

The explored caves are situated geographically in the northernmost part ot the tropical karst area, but in fact they do not seem to correspond to this type of karst. They are relatively short – up to a couple of hundreds of meters. They lack the big volumes and the abundant presence of speleothems, which are the typical parameters of the tropical karst phenomena. This fact can be explained with the altitude (1000 – 2100 m) – a temperature factor which in combination with the latitude (~ N 25º) takes them out of the zone of active tropical karst genesis. We must not forget that the most favourable temperature for dissolution and redeposition of Ca(Mg)CO3 is approximately 25º C.

Some of the explored caves, however, have a more complex or different of the karst genesis. Here we attach their more detailed descriptions:

1. Bian Fu Dong - 1 (Bat Cave N° 1), near to Bin Men village.

N 25.09549°, E 98.82200°, Alt. 810 m.

The cave is situated at the foot of the western slopes of Nujiang (Salween) River. It is a straight-line gallery without branches, with approximative length of 1500 m and light dip. In fact, this is an underground brittle unstick, tracking down inside a plastic shear zone. The zone is 50-80 м wide. It progress in N-S direction (350°-170°), and slips down to east with mainly 45° dip. This zone represents one of the westernmost subparallel ductile faults that take part of the Nujiang Fault System.

The mylonites in the zone are developed above a monotonous mudstone matrix. They contain new-formed muscovite, sericite and chlorite that indicate greenschist facies conditions of their formation. The muscovite sheets reach to 1 cm width. They represent the most abundant mineral in the greenschist paragenesis. The observed shear-sense criteria indicate that the zone was affected of subhorizontal dextral strike-slip plastic shear.

This cave has nothing to do with the karst. It is purely tectonic phenomenon. There are not carbonate rocks in the area. In some places inside the cave, there are fractures into the rocks with cooling or dribble water inside, slightly enriched in CO2, which precipitate some centimetric stalactites.

2. Bian Fu Dong - 2 (Bat Cave N° 2), near to Hemu village.

N 24.98442°, E 98.80489°, Alt. 1296 m.

The cave is situated in the inner part of Gaoingong Shan Range. It is a single meander-like gallery that is bifurcated in its far end part. The total lenght of the cave is 429 m, and the denivelation is -24,4 m. It is develloped in marbleized limestones, immidiately below their contact with black mudstones.

The contact itself is tectonic - a ductile thrust fault develloped in the conditions of greenschist facies. The limestones and the mudstones are strongly mylonitized. In the first couple of meters below the contact, the limestones contain many lenses and levels from the mudstones. The mylonitic foliation from the two sides of the contact is subparallel to the stratification. It slips down to East with mainly 50° dip. The mineral lineation climbs to NE.

The cave is typical karst phenomenon, formed through the system of brittle fractures that inherited the ductile fault.

3. Djin Dong (lava tube), near to Tengchong town.

N 25.03170°, E 98.43969°, Alt. 1642 m.

The cave is situated in Tengchong volcanic district, located in southern China near the border with Myanmar (Fig. 7). It is 729 m long, and has denivelation -24,3 m / + 69,8 m. It is a typical lava tube formed by a Pleistocene basaltic-andesitic lava flow. A thick layer of mud covers the floor of the tube. From some fractures in the ceiling dribble slightly enriched in CO2 water, which precipitate some centimetric stalactite embryos.

The volcanism of the Tengchong Rift is predominantly andesitic, Pliocene to Pleistocene in age. It is related to the eastward subduction of the Burma microplate under the Eurasian plate near the northeastern side of the collision zone between the Indian and Eurasian Plates (Zhijie and Guoying, 1986).

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