North-directed Triassic nappes in Northeastern Vietnam (East Bac Bo)

A careful re-examination of the previous tectonic descriptions and the acquisition of new structural and kinematic data have been undertaken in Northeastern Vietnam. The structure of the area consists of a system of slightly metamorphosed but ductilely deformed nappes, including recumbent folds, formed during the Triassic, prior to the unconformable deposition of the Upper Triassic terrigenous sediments. These results confirm the previous interpretation of “preyunnanaises nappes”, represented by Middle-Upper Paleozoic foliated limestone resting through a flat mylonitic contact over an intermediate “Song Mien unit” (Deprat, 1915), the latter made essentially of deformed Lower Triassic sedimentary and volcanic formations, which previously filled the Song Hien rift (Bourret, 1922a and Bourret, 1922b). In the external part of the belt, the Triassic strata conformably overlie slightly deformed and unmetamorphosed Upper Paleozoic rocks, forming an autochthonous domain. Alternatively, the same Triassic succession can be seen resting tectonically through décollement zones, directly over the Middle Paleozoic marbles, to form a distinct but subsidiary allochthonous unit. Our data show that the Song Chay orthogneiss and its Lower Paleozoic sedimentary country rocks belong to the main nappe. They also demonstrate that this nappe was transported to the N–NE. A newly discovered ophiolitic mélange of supposed oceanic nature and Triassic age, lying along the Song Chay Fault, North of the Nui Con Voi, could hypothetically represent the rooted zone of the nappe. A Tertiary overprinting event, in particular accommodated by strike-slip movements, likely accounts for the present orocline of NE Vietnam.

Research highlights
► Ductile nappes of Triassic age, involving the Song Chay massif, characterizes NE Vietnam.
► Transport direction is to the N–NE.
► Origin of the nappes is located along the Song Chay Fault.
► Along this fault a zone of mélange is interpreted as a Triassic suture.