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[1] Discrete scarps that are created or reactivated during large earthquakes are a locus of concentrated hazard. A number of the coseismic scarps activated in the 1999 Chi-Chi earthquake are actually fold scarps, which display several types of ground deformation characterized by localized folding and are distinct from classic fault scarps, which form by a fault cutting the surface. This paper documents and analyzes fold scarps that formed or reactivated in the 1999 Chi-Chi Taiwan earthquake. Our results show the Chi-Chi fold scarps can be generally divided into two types: (1) those associated with folding ahead of the tip of a blind thrust fault at shallow depths and (2) those associated with folding by kink band migration above fault bends at substantial depths ranging from $0.8 to 5 km). The previously published trishear model can be applied to model the former type, while a new curved hinge kink band migration model is provided to describe the behavior of the latter type. A key feature of fold scarps of the second type is that hinge zones are typically wide (25-100 m) relative to the displacement in a single earthquake (1-10 m), which exerts a significant control on fold scarp morphology and evolution. Because the coseismic strains of both types of fold scarps display relatively wide deformation zones (10-100 m) relative to fault scarps, wider set-back zones might be appropriate from a public policy point of view to alleviate the risk to structural damage and collapse resulting from permanent ground deformation.

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The Yuli metamorphic belt has been the topic of petrological and geochronological studies for over 40 years and has been interpreted as a Cretaceous mélange. Our study utilizes zircon U‐Pb dating of schist and exotic blueschist blocks in the Yuli belt. These new ages indicate that these metamorphic rocks are actually middle Miocene in age and may represent the deeper structural levels of an accretionary prism. Several distinctive detrital zircon U‐Pb age populations are recognized from 14 siliceous schists of mélange‐hosted rocks that are similar in age population to the Cretaceous, Eocene‐Oligocene, and Miocene strata of Taiwan. The wide range of ages is interpreted as a product mixing of various sedimentary strata prior to metamorphism. Three blueschists of a volcanic‐arc protolith enclosed within the host rocks yield crystallization ages of 15.4 ± 0.4, 15.5 ± 0.3, and 16.0 ± 0.2 Ma based on zircon U‐Pb dating. In consideration of the new data regarding the Cretaceous‐Miocene host rocks and the middle Miocene exotic blueschist blocks, it strongly suggests that the Yuli belt formed at deeper levels of an accretionary wedge during subduction of South China Sea oceanic crust at the middle‐late Miocene. Subsequently, the rapid uplift of the metamorphic belt was probably related to doubly vergent wedge extrusion due to the Pliocene arc‐continent collision.