The widespread occurrence of low-angle normal faults in a rift setting: Review of examples from Thailand, and implications for their origin and evolution

Abstract

At least 31 low-angle (<35° dip at the time of initiation), high-displacement (>1km) Cenozoic normal faults are identified on seismic reflection data onshore and offshore Thailand. Although some faults have been moderately rotated to lower angles, pre-rotation dips still indicate formation at low angles (about 25° and 35°). The dominant east-dip of low initial fault dips suggest that pre-existing fabrics controlled the fault dip direction. In the Mergui Basin patches of gently dipping basement reflections on seismic reflection data support this observation. Well data points to a pre-rift basement with a widespread component of slaty, phyllitic, and chlorite-schist lithologies that are associated with significant strength anisotropy. The subduction zone setting of SE Asia, with a high fluid flux into the crust, associated with high levels of CO2, has parallels with the setting for LANFs in Italy, where it is suggested high pore fluid pressures played an important role in LANF reactivation. In cross-section most LANFs curve from a high-angle in sedimentary section to planar or stepped in basement until they reach the brittle–ductile transition zone. In a few cases LANFs have a convex-up geometry. Some long, segmented faults are composed of mixed regions of high- and low-angle segments. The local mix of high and low-angle faults indicates that the principal stresses probably were vertical and horizontal during fault formation. Any rotation of stresses would have to be local not regional. LANFs in the Mergui Basin have very high displacement gradients (as low as 2:1) indicating the same fault segments (some 20–30km long) have repeatedly slipped, while displacements can decline dramatically at key discontinuities. The LANF segments are inferred to be very weak, and probably slipped mostly aseismically, while adjacent, lower displacement high-angle segments were probably seismically active. Hence, in areas of mixed high- and low-angle fault segments seismicity may erroneously suggest that high-angle normal faults are the dominant fault type. The presence of such a high concentration of low and high displacement LANFs in a non-metamorphic core complex setting demonstrates that LANFs can form initially at a low-angle, that isostatic rotation of initially high-angle faults to a low-angle is not necessary or appropriate to explain the faults, and that low-angle normal faults are not just features associated with orogenic collapse.