HR: 0800h
AN: T41C-1221    [Abstracts]
TI: Along Strike GPS and Seismic Constraints on Extension in the Eastern Central Range, Taiwan
AU: * Gourley, J R
EM: jonathan.gourley@uconn.edu
AF: University of Connecticut, Department of Geology and Geophysics, 354 Mansfield Rd. U-2045, Storrs, CT 06269 United States
AU: Byrne, T
EM: tim.byrne@uconn.edu
AF: University of Connecticut, Department of Geology and Geophysics, 354 Mansfield Rd. U-2045, Storrs, CT 06269 United States
AU: Wu, F T
EM: wu@geol.binghamton.edu
AF: Binghamton University, Department of Geological Sciences and Environmental Studies, P.O. Box 6000, Binghamton, NY 13902-6000 United States
AU: Chan, Y
EM: yuchang@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road Section 2, Nankang, Taipei, 115 Taiwan
AU: Yu, S
EM: yusb@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road Section 2, Nankang, Taipei, 115 Taiwan
AU: Kuo, L
EM: kuo@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road Section 2, Nankang, Taipei, 115 Taiwan
AB: Synorogenic extension is well documented in many active mountain belts around the world and has gained recent attention in the formation of the Taiwan arc-continent collision (e.g. Crespi et al., 1996; Yu et al., 1997; Kao and Chen, 2000; Wang and Hung, 2002; Bos et al., 2003). Available GPS, seismic and geologic data sets and new GPS data from the Hengchun Peninsula, Taiwan illuminate a pattern of extension east of the Taiwan mountain crest. Kinematic data extracted from the BATS data set suggest a change in the extensional mechanism from gravitational collapse in the north to lateral extrusion in the south. We surveyed GPS monuments in southern Taiwan between January 2000 and November 2002 that can be divided into two data sets: five sites west of the mountain crest and three sites east of the crest. West of the crest, the five monuments have an average velocity of 51 mm/yr westward, whereas east of the crest the average velocity of three monuments is 40 mm/yr. This decrease in velocity from west to east indicates a zone of extension just east the mountain crest. The eastern monuments also have higher horizontal north component velocities than the western monuments. Extension in the Hengchun Peninsula correlates with similar patterns of extension along strike and together define a zone of extension east of the crest that extends nearly the entire length of the orogenic belt. The geometry and kinematics of faults in the eastern Central Range extensional zone, however, show a change from north to south. In northern Taiwan (in the vicinity 23.5\deg N) the geometries of major fault structures are interpreted from patterns of seismicity prior to and after the 1999 Chi-Chi earthquake, using, in part, the collapsing method of Bossu (2000). These data suggest two north-striking and near vertical fault zones: a post-Chi-Chi, west-side down normal fault zone just east of the mountain crest and extending from about 5 to 15km and a pre-Chi-Chi, west-side up reverse fault east of the normal fault and near the Longitudinal Valley that extends from 15 to 30 km. Focal mechanisms for the reverse faults (from the BATS database) show P-axes plunging gently NW and T-axes plunging near-vertical. Focal mechanisms for the extensional faults show P- and T-axes that are inverted relative to the solutions for the reverse faults. This inversion to extension following the Chi-Chi earthquake is interpreted to represent a period of west-side-down gravitational collapse along the fault east of the topographic crest. To the south, in the vicinity of the Southern Cross Island Highway, the fault geometries are less organized and pre- and post-Chi-Chi patterns have not been recognized. The available kinematic data are also only partially consistent with the data to the north. Although extensional earthquakes dominate the region east of the crest the associated T-axes plunge gently SW. This change in the T-axes orientations during extensional events from steeply plunging in the north to gently southwest plunging in south is interpreted to reflect the lateral flow of material as the oblique collision propagates towards the South China Sea. In this southern zone of extrusion extension appears to be accommodated by a variety of structures, including oblique strike-slip and normal faults.
DE: 7230 Seismicity and seismotectonics
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8109 Continental tectonics--extensional (0905)
DE: 1206 Crustal movements--interplate (8155)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting