HR: 15:15h
AN: T13F-07    [Abstracts]
TI: Millennial Slip Rate of the Longitudinal Valley Fault, a Major Element of Taiwan's Tandem Suturing
AU: * Shyu, J H
EM: jbhs@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Sieh, K
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Avouac, J
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Chen, W
AF: Department of Geosciences, National Taiwan University, Taipei, 106 Taiwan
AU: Chen, Y
AF: Department of Geosciences, National Taiwan University, Taipei, 106 Taiwan
AU: Lin, C
AF: Central Geological Survey, MOEA, Chungho, 235 Taiwan
AB: The Longitudinal Valley fault is a principal structure in the ongoing tandem suturing of Taiwan. We have used a fluvial terrace to show that over the past 4,000 years the dip-parallel component of slip on this left-lateral reverse fault has accumulated at a rate in excess of 30 mm/yr. Deformation of the terraces in the hanging-wall block, above the fault, allows us to infer the geometry of the fault plane. The island of Taiwan is being created by a tandem suturing and disengagement of the Eurasian continental margin, the Luzon volcanic arc, and an intervening continental sliver. Along the eastern suture, marked by the narrow, north-south Longitudinal Valley, the Luzon arc is accreting to the metamorphic core of the island, forming the Coastal Range of Taiwan above the east-dipping Longitudinal Valley fault. Near the middle of the range, the antecedent Hsiukuluan River flows from west to east through the range. Multiple generations of fluvial terraces along the course of the river enable the calculation of uplift rates for the Coastal Range. Combination of these with a model of the fault geometry constrains the millennial slip rate of the Longitudinal Valley fault. Radiocarbon dating of charcoal from the terrace deposits shows that the terraces were formed by the incision of the river during the past 8,000 years. A 4,000-year-old terrace that is preserved locally from the fault to a point about halfway through the Coastal Range forms a broad anticline. Assuming that river incision keeps pace with rock uplift, the maximum rock uplift rate is about 30 mm/yr, near the village of Chimei, approximately 4 km east of the fault. The rate decreases to about 20 mm/yr near the fault. Previously published work shows that the uplift rate is only about 6 mm/yr farther east, near the coastline. Using this anticlinal pattern of local rock uplift rate and variations in bedding dips along the Hsiukuluan River, we can infer the subsurface geometry of the Longitudinal Valley fault and a slip rate of about 35 to 42 mm/yr for the past 4,000 years. The horizontal component of the shortening, perpendicular to the fault, would be 17 to 30 mm/yr, comparable to the rate of horizontal shortening across the region measured by GPS. This first constraint on the long-term slip rate of the Longitudinal Valley fault demonstrates that most of the shortening between the Luzon arc and the Central Range of Taiwan is concentrated on this structure. Rates of slip on the Central Range thrust and any offshore structures must be dwarfed by the slip rate of the Longitudinal Valley fault. This demonstrates that over the millennia, the Longitudinal Valley fault embodies most of the seismic hazard along Taiwan's eastern suture.
DE: 7223 Seismic hazard assessment and prediction
DE: 8015 Local crustal structure
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting