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.

The closure of Palaeo-Tethys in Eastern Myanmar and Northern Thailand: New insights from zircon U–Pb and Hf isotope data

“…The discovery of strongly deformed ortho-and paragneisses, principally outcropping at Doi Inthanon and Doi Suthep, are interpreted to represent the exposure of a deeper, basement complex. Later work led to the region being termed the "Inthanon Zone", and the interpretation that these migmatitic outcrops at Doi Inthanon and Doi Suthep represented metamorphic core complexes exposed through low angle detachment faulting during lateral extension, similar to those found within the Basin and Range province of southwestern North America (Barr and Macdonald, 1991;Macdonald et al, 1993;Dunning et al, 1995;Rhodes et al, 2000;Morley, 2009).…”

Low-temperature thermochronologic constraints on the kinematic histories of the Castle Cliffs, Tule Springs, and Mormon Peak detachments, southwestern Utah and southeastern Nevada

“…According to such theory, only steep normal faults should form within the brittle part of the crust and faults that dip <30° should be frictionally locked (Anderson, 1942(Anderson, , 1951. Consequently, slip on LANFs can only occur by invoking some special circumstance, such as reactivation of a preexisting weakness (e.g., Morley, 2009), rotation of the stress field (e.g., Yin, 1989), and/or high pore-fluid pressures (e.g., Collettini and Barchi, 2002). The lack of convincing seismological evidence for large earthquakes on LANFs is also frequently used as an argument against their mechanical feasibility (Jackson, 1987;Jackson and White, 1989;Thatcher and Hill, 1991;Collettini and Sibson , 2001).…”

Did Oligocene crustal thickening precede basin development in northern Thailand? A geochronological reassessment of Doi Inthanon and Doi Suthep

“…However, the complexes at Doi Inthanon and Doi Suthep are overlain by a cover of lower-grade Palaeozoic metasedimentary rocks, and later work led to the interpretation that they represented Cordilleran-type metamorphic core complexes exposed through low angle detachment faulting during lateral extension , Dunning et al, 1995, Morley, 2009aand Rhodes et al, 2000. In this scenario they find a parallel within the core complexes of the Basin and Range province of southwestern North America (e.g., Wernicke, 1992).…”

“…In this model the extension phase is highly protracted (~ 25 My), and overlaps with a period of regional transpressional deformation and highgrade metamorphism. Morley, 2009a andMorley, 2009b suggested as an alternative that the early top to the east shear could be related to thrusting. Reassigning an early Miocene age to movement on the detachment, as implied by a late Eocene-Oligocene metamorphic event, has obvious implications for any model for the development of the Chiang Mai Basin.…”