Lateral extrusion and tectonic escape in Ilan Plain of northeastern Taiwan Angelier, J., Chang, T.Y., Hu, J.C., Chang, C.P., Siame, L., Lee, J.C., Deffontaines, B., Chu, H.T, Lu, C.Y., Does extrusion occur at both tips of the Taiwan collision belt? Insights fromactive deformation studies in the Ilan Plain and Pingtung Plain regions, Tectonophysics, 125(1-3), 39-72, 2008. Hou, C. S., Hu. J. C., Chen, Y. G., Chen, C. L., Cheng, L. W., Tang, C. L., Huang, S. H. and Lo, C. H., The crustal deformation of the Ilan Plain acted as a westernmost extension of the Okinawa Trough, Tectonophysics, 466, Issues 3-4, 344-355, 2009.
Outline Introduction General structure Present-day kinematics from GPS studies The present-day seismotectonic stress Discussion and conclusions
Introduction extension subduction collision GPS data lateral extrusion Ilan plain is located at the northeastern Taiwan and is considered as a delta fan formed as a back-extension of Okinawa Trough. (Sibuet, et al., 1998;Clift, et al., 2008)
The crustal deformation in Ilan Plain of northern Taiwan located in a transition zone of collision and surrounding subduction. Tectonic extrusion describes the lateral motion of structural units that move toward a weaker domain with respect to the mountain belt, in response to collision-induced shortening. We are combining two different approaches to investigate the geometrical and mechanical relationships between the Taiwan collision belt and the adjacent subduction zones.
Tectonic framework and main structural units of the Taiwan collision zone
General structure N S extension dominates near the western tip of the Okinawa Trough (Ilan Plain and adjacent offshore areas), whereas E W extension prevails in northernmost Taiwan (Taipei basin and the Tatung volcano field). (Lu et al., 1995)
Morphology and generalised geology of the Ilan Plain 24.7N N25E
Horizontal velocity vectors based on GPS studies Shangai Paisha Add a WNW-directed displacement vector of about 1 cm/yr small Not a rigid rotation! Large from 1992 to 1996 by Hsu from October 2002 to December 2005 By Hu et al
Seismic activity in and around the Ilan Plain area Grey : 1992~1996 White : 2002~2005 Thick : 0 40 km in depth Thin : deeper earthquakes M=6.8 in 2002 June 5th, 1994 and June 25th, 1995
GPS results Data collection From Ilan GPS Network 2002 ~ 2006 (Hou et al.,2009) 34 Stations 4 times dual-frequency geodetic receivers each station was usually active during more than two sessions, each session being composed of 6 14 h of GPS data acquisition from all available satellites rising higher than a 15 elevation angle. sampling interval 15 s.
GPS results The observed N S and E W displacement as a function of time from 2002 to 2006 for the benchmarks in study area.
1.9 mm/yr ~ 7.5 mm/yr 38 o ~ 90 o 5.8 mm/yr ~ 21.8 mm/yr E - W tensile force trench retreat related to the suction force at the Ryukyu trench From pure compression to transtensional regime Compressional force Philippine Sea plate 45.6 mm/yr 144 o Lateral extrusion process towards mechanically weak domain
Strain rate E-W INCREASE NW-ES
Strain rate The largest extensional rate 2.66 μstrain/yr in 147 o shortening rate 1.28 μstrain/yr in 47 o in the perpendicular direction Transtensional deformation mode!
Spatial variation of crustal strain of Ilan Plain Rotation rate field anticlockwise 0.26-0.69μrad/yr clockwise 1.3μrad/yr left-lateral shear clockwise 1.17-1.45μrad/yr
The present-day seismotectonic stress 115 earthquakes during July 1995 to June 2003 magnitude 3.4 to 6 depth 13 and 118 km
Strike-slip NW-SE extension normal NW-SE extension normal N-S extension normal E-W extension reverse WNW-ESE compression Well fit the average trend of compression across most of the Taiwan belt.
Transtension! a a extension related to normal faulting mechanisms b b extension/compression related to strike slip mechanisms c c extension/compression indicated by geodetic data average trends of compression
Discussion and conclusions Because of active subsidence and highrate sedimentation, the surface expression of the normal faults in the Ilan Plain is subdued. Permutation between maximum and intermediate principal stresses, σ1 and σ2 respectively, a common phenomenon in brittle tectonics.
Discussion and conclusions GPS studies provide a picture of the deformation at the surface, which does not necessarily reflects the deformation at depth in a direct way. Displacement & strain & Seismotectonic stress
Discussion and conclusions increase in elongation rates from West to East closer to the Okinawa Trough W-E NW-SE closer to the Ryukyu arc and trench
Discussion and conclusions The active deformation of the Ilan area is clearly affected by two extensional regimes. The expression of the lateral extrusion caused by the Taiwan orogeny NW-SE extension Okinawa extension N-S E-W extension
Discussion and conclusions close left-lateral shear
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