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Complex Cainozoic Rifting and Pulsed Inversion in the Northern Song Hong Basin, Vietnam

Abstract

The evolving Eocene to Recent regional stress pattern, including the geodynamic development of the Red River Shear Zone, is reflected in the structural style of the northern Gulf of Tonkin. During the Eocene – Oligocene, intense NW- to NE-trending rift-faulting suggests a dominating N–S-trending tensile stress associated with left-lateral transtension across the southern and offshore part of the Red River Shear Zone. During the later Oligocene to earliest Neogene time, inversion, dominantly taking place across NE-trending faults, suggests a change to regional NW–SE-compression. Continued inversion taking place throughout the Miocene indicate that this stress pattern continued into the Neogene. Towards the end of the Miocene a slight counter-clockwise reorientation of the stress pattern resulted in E–W-compression and N–S-extension which caused NW-transpression and moderate E–W-trending normal faulting. The structural style observed throughout Eocene to Miocene time is thus compatible with left-lateral motion across the Red River Shear Zone peaking during the Paleogene. The Plio-Pleistocene is generally considered as a period of tectonic quiescence. Nonetheless, compressional inversion continues to the present on the NE-trending Bach Long Vi structure offshore. At the same time, tens of kilometres of right-lateral displacement has been inferred across the onshore part of the Red River Shear Zone. This is difficult to reconcile with the lack of Plio-Pleistocene faulting along the offshore part of the Red River Shear Zone. We suggest that up to tens of km of the lateral movement could have been accommodated within a number of NE-trending major fault strands onshore similar to that in the Bach Long Vi area following the Miocene.