Geometry and evolution of low‐angle normal faults (LANF) within a Cenozoic high‐angle rift system, Thailand: Implications for sedimentology and the mechanisms of LANF development

Abstract

At least eight examples of large (5–35 km heave), low‐angle normal faults (LANFs, 20°–30° dip) occur in the Cenozoic rift basins of Thailand and laterally pass into high‐angle extensional fault systems. Three large‐displacement LANFs are found in late Oligocene–Miocene onshore rift basins (Suphan Buri, Phitsanulok, and Chiang Mai basins), they have (1) developed contemporaneous with, or after the onset of, high‐angle extension, (2) acted as paths for magma and associated fluids, and (3) impacted sedimentation patterns. Displacement on low‐angle faults appears to be episodic, marked by onset of lacustrine conditions followed by axial progradation of deltaic systems that infilled the lakes during periods of low or no displacement. The Chiang Mai LANF is a low‐angle (15°–25°), high‐displacement (15–35 km heave), ESE dipping LANF immediately east of the late early Miocene Doi Inthanon and Doi Suthep metamorphic core complexes. Early Cenozoic transpressional crustal thickening followed by the northward motion of India coupled with Burma relative to east Burma and Thailand (∼40–30 Ma) caused migmatization and gneiss dome uplift in the late Oligocene of the core complex region, followed by LANF activity. LANF displacement lasted 4–6 Ma during the early Miocene and possibly transported a late Oligocene–early Miocene high‐angle rift system 35 km east. Other LANFs in Thailand have lower displacements and no associated metamorphic core complexes. The three LANFs were initiated as low‐angle faults, not by isostatic rotation of high‐angle faults. The low‐angle dips appear to follow preexisting low‐angle fabrics (thrusts, shear zones, and other low‐angle ductile foliations) predominantly developed during Late Paleozoic and early Paleogene episodes of thrusting and folding.