Cenozoic paleostress evolution in south central Vietnam: Implication for changing dynamics of faulting along the eastern Indochina continental margin

Abstract

The tectonics of the Indochina continental margin before and after the Neogene-Quaternary basalt eruption is poorly understood. The analysis of 1500 striated faults observed in different rocks, mostly from volcano-plutonic rocks with ages ranging from Cretaceous to Quaternary, at more than 140 locations in south central Vietnam by direct inversion method allowed us to determine four paleostress regimes that affected eastern Indochina from Oligocene up to present day. The first tectonic paleostress regime with Oligocene age was responsible for the sinistral and dextral motions along the NW-SE and NE-SW striking faults respectively. This stress state is typical for a strike-slip tectonic regime with maximum principal stress (σ1) and minimum principal stress (σ3) axes oriented nearly in E-W and N-S direction and both plunging at average angle of 80 and 100, respectively. The paleostress ellipsoid shape ratio Φ (Φ = (σ2 − σ3)/(σ1 − σ3)) ranges from 0.23 to 0.77 with an average of 0.50 ± 0.15. The subsequent strike-slip tectonic stress state prevailing from the Late Oligocene to very Early Miocene, was characterized by σ1 and σ3 axes trending in NNW-SSE and ENE-WSW and plunging at 120 ± 90, and 120 ± 130, respectively. The paleostress ellipsoid shape ratio Φ ranges from 0.12 to 0.88 with a 0.52 ± 0.23 average. Under this paleostress regime, the N1100 to N1400 and N1600 to N200 striking faults moved with dextral and sinistral movements, respectively. The third paleostress state with assigned Early Miocene to Late Miocene age was responsible for dextral and sinistral motions along N1400 to N1600 and N200 to N400 striking faults, coeval with emplacement of N-S striking basaltic dykes. This pattern shows consistency and homogeneity of the σ1 and σ3 axes in N-S trend, plunging at 120 ± 90 and σ3 in E-W trend, plunging at 130 ± 110, respectively. The paleostress ellipsoid shape ratio Φ varies from 0.41 to 0.89 with an average of 0.53 ± 0.16. The fourth tectonic stress regime occurring since the Late Miocene to the present day is characterized by a consistency of sub-horizontal NNE striking σ1, plunging 110 ± 100 and ESE striking σ3, plunging at 130 ± 11, and an average of ellipsoid shape ratio Φ = 0.45 ± 0.24. Under the latest tectonic stress state, the N-S and N450 to N600 striking faults continue to move as dextral and sinistral faults, respectively. The first three paleostress regimes have evolved clockwise, and were induced by the force exerted from the eastern Himalayan syntaxis during the northward Indian plate motion, while the fourth tectonic stress regime is most likely linked to the force originated from the Sunda subduction.