Late Tertiary tectonics of the Red River Fault Zone: Structural evolution of sedimentary rocks

The Red River Fault Zone (RRFZ), lying between eastern Tibet and the Tonkin Gulf, is the most conspicuous geological and geomorphological discontinuity in Southeast Asia. Despite different opinions, the tectonics of this zone can be explained best in terms of Oligocene to Recent lateral escape related to the India/Eurasia collision and opening of the East Vietnam Sea (South China Sea).The widely accepted models of the RRFZ tectonics are largely based on the analysis of metamorphic rocks, offshore data and geomorphological results. According to these models, the late Tertiary to Recent structural development of the RRFZ comprised three successive stages: (1) 34 Ma until 17 Ma – sinistral shift coinciding with an initial event of rapid cooling and uplift; (2) 17 Ma until 5(?) Ma – slow cooling and uplift; and (3) 5(?) Ma until present day – dextral shift coinciding with a second event of rapid cooling and uplift.The main drawback of these results is the paucity of information on the kinematics of the RRFZ in the period between 17 and 5 Ma. Therefore, reconstruction of the structural evolution of the RRFZ during that period is the main target of this paper.Structural analysis of the Miocene strata was performed at 76 exposures located along the entire Vietnamese segment of the RRFZ (ca. 200 km) has concluded that the Miocene succession was syn-sedimentary folded in a stress field with maximum horizontal compression axis (σ1) oriented NE–SW. The folding was of a thin-skin type affecting only the Miocene succession and not its basement. This folding was followed by three successive stages of brittle deformation: (i) jointing, clastic dyking, and strike-slip and reverse faulting, which took place in a stress regime typified by σ1 oriented W–E to NE, roughly perpendicular to the RRFZ; (ii) normal faulting, which occurred due to N–S oriented extension; and (iii) strike-slip faulting taking place in a stress field with σ1 oriented WNW–ESE to ENE–WSW, roughly compatible with sinistral shift along the RRFZ.