Regional Geology
The area encountered regional metamorphism in Silurian to Devonian. The mafic rocks of Precambrian and marine sediments of Cambrian to Silurian were metamorphosed to mainly pumpellyite-actinolite facies to green schist facies. After Devonian no clear evidence of formation of metamorphic minerals and metamorphic textures are observed. The metamorphic events in the area, therefore, are restricted in a period of after the formation of Silurian marine sediments and before formation of Devonian marine sediments.
The three periods of tectonic movements in the area, Silurian to Devonian, Devonian to Carboniferous and Permian to Triassic, were identified and each of which, respectively correspond to Caledonian, Variscan, and Indosinian periods of tectonic movements. The characteristic features of each of the tectonic movement in the Attapeu Area are summarized below.
- I. Silurian to Devonian (Caledonian): Occurrences of pumpellyite-actinolite facies to green schist facies metamorphic rocks.
- II. Devonian to Carboniferous (Variscan): Occurrences of pumpellyite-actinolite facies metamorphic rocks.
- III. Permian to Triassic (Indosinian): Occurrences of fractures zones characterized by mylonite and cataclasite.