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Abstract

Yue, Li-Fan, John Suppe, and Jih-Hao Hung, 2011, Two contrasting kinematic styles of active folding above thrust ramps, western Taiwan, in K. McClay, J. H. Shaw, and J. Suppe, eds., Thrust fault-related folding: AAPG Memoir 94, p. 153186.

DOI:10.1306/13251337M943431

Copyright copy2011 by The American Association of Petroleum Geologists.

Two Contrasting Kinematic Styles of Active Folding Above Thrust Ramps, Western Taiwan

Li-Fan Yue,1 John Suppe,2 Jih-Hao Hung3

1Department of Geosciences, Princeton University, Princeton, New Jersey, U.S.A.; Present address: Chevron Energy Technology Company, Houston, Texas, U.S.A.
2Department of Geosciences, National Taiwan University, Taipei, Taiwan; Also at: Department of Geosciences, Princeton University, Princeton, New Jersey, U.S.A.
3Department of Earth Sciences, National Central University, Jhongli, Taiwan

ACKNOWLEDGMENTS

We are grateful to Yue-Gau Chen of the National Taiwan University for providing us with DEM data. We are extremely grateful to PetroChina for hosting the International Conference on Theory and Application of Fault-Related Folding in Foreland Basins, Beijing, China, 2005, which stimulated the publishing of this chapter and the volume. Yue thanks Princeton University for providing support during 2001–2007. Special thanks to Ken R. McClay for the effort to edit the volume for publication. The trishear interpretation in Figure 16A2 was produced using the Fault/Fold program of Rick Allmendinger.

ABSTRACT

Two adjacent active thrust ramps in western Taiwan show contrasting hanging-wall structural geometries that suggest different kinematics, although they involve the same stratigraphic section and basal detachment. The Chelungpu thrust shows a classic fault-bend folding geometry, which predicts folding by kink-band migration without limb rotation, whereas the hanging wall of the Changhua thrust shows the characteristic geometry of a shear fault-bend folding, which predicts a progressive limb rotation with minor kink-band migration. We test the kinematic predictions of classical and shear fault-bend folding theories by analyzing deformed flights of terraces and coseismic displacements in the 7.6 moment magnitude scale Chi-Chi earthquake. In particular, differences in terrace uplift across active axial surfaces are used to show that the assumptions of classical fault-bend folding are closely approximated, including constant fault-parallel displacement, implying conservation of bed length, and hanging-wall uplift rates that are proportional to the sine of the fault dip. This provides a basis for precise determination of total fault slip because the formation of each terrace, combined with terrace dating, gives long-term fault-slip rates for the Chelungpu thrust system. Even the coseismic displacements of 3 to 9 m (10 to 29 ft) in the Chi-Chi earthquake are approximately fault parallel but have additional transient components that are averaged out over the time scale of terrace deformation, which represents 10–100 large earthquakes. In contrast, terrace deformation in the hanging wall of the Changhua thrust ramp shows progressive limb rotation, as predicted from its shear fault-bend folding geometry, which combined with terrace dating allows an estimation of the long-term fault-slip rate of 21 mm/yr (0.83 in./yr) over the last 31 ka. A combined shortening rate of 37 mm/yr (1.46 in./yr) is obtained for this part of the western Taiwan thrust belt, which is about 45% of the total plate-tectonic shortening rate across Taiwan. The Changhua shear fault-bend fold ramp is in the early stages of its development with only 1.7 km (1.06 mi) total displacement, whereas the Chelungpu classical fault-bend fold ramp in the same stratigraphy has nearly an order of magnitude more displacement (sim14 km [8.7 mi]). We suggest that shear fault-bend folding may be favored mechanically at low displacement, whereas classical fault-bend folding would be favored at large displacement.

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