Geometric Evolution of the Chelungpu Fault, Taiwan: The Mechanics of Shallow Frontal Ramps and Fault Imbrication

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Journal of Structural Geology







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The September 21, 1999 Mw=7.6 earthquake in Taiwan ruptured along the north–south-trending, east-dipping Chelungpu thrust fault that bounds the western foothills of the Taiwan fold-and-thrust belt. The near-surface (<4000 m deep) fault geometry changes from a simple wedge above a footwall ramp in the southern region, to a ramp–flat, bed-parallel geometry in the northern Chelungpu region. Rupture characteristics also varied between the northern and southern regions; higher frequency accelerations but lower velocity shaking was experienced in the southern region compared with the northern region. The hanging-wall flat, bed-parallel geometry of the northern Chelungpu region is the result of recent (~50 ka) fault migration ~1 km eastward away from the Sanyi fault trace. By contrast, the southern part of the Chelungpu fault is confined to its original trace that appears to have been active for much of the Quaternary. We show that this ‘out-of-sequence’ fault migration into the hanging wall in the northern region occurred due to the mechanics of the fault geometry at the leading edge of the thrust sheet where the shallow emergent thrust sheet is elongated due to the near-surface, bed-parallel flat. A simple mechanical model shows how resisting forces in the wedge-shaped hanging wall, which overrides a thick conglomerate sequence in the northern region, exceeds the tectonic load driving the slip of the thrust at the base of the wedge. In response, the fault forms a hinterland imbricate that shortens the hanging wall thrust sheet, reduces the volume of the emergent hanging-wall wedge, and thus reduces the resisting force. The southern region has a simple-wedge geometry that produces resisting forces in the hanging wall, to depths of 4000 m, that are less than or equal to the tectonic driving forces, allowing the thrust sheet to move without internal deformation or imbrication. This investigation of the Chelungpu fault indicates that hindward migration of an individual thrust fault can occur simultaneously with foreland-progression of the fold-and-thrust belt, given a shallow (<4000 m) footwall flat within the emergent thrust.