Abstract
The ongoing orogeny in Taiwan is a result of the collision of the PhilippineSea and the Eurasian plates.While the structure of the continentalcrust on the Eurasian plate(EP)is now mostly known,that of the oceaniccrust on the Philippine Sea plate(PSP)is not well mapped.Using offshore-onshore refraction data,collected during the R/V Ewing cruise of 1995,weinvestigate the nature of the transition between the EP and the PSP in thevicinity of the southern Coastal Range of Taiwan.The data were producedby the air-gun array of the R/V Ewing along a WNW-ESE trending line offthe coast of central Taiwan(MCS/OBS line 23).The refracted P waveswere collected by 19 RefTek recorders with L-28 sensors placed along thesouthern cross-island highway of Taiwan,which is the onshore extension ofline 23.Because of high noise levels in the Coastal Plain,we were only ableto retrieve usable data from 11 stations located from the east coast throughthe Central Range.We performed forward modeling of the first arrivingwaves at these stations;our strategy was to search for the simplest modelthat fitted all the data.Our results indicate that the crust thickens graduallytoward Taiwan,from about 9-12km thick in the Huatung basin to 15-18km thick off the eastern coast of Taiwan,with a 4-8 degree dip of theMoho.Continuing westward,the crust thickens more rapidly to 27-32kmthick under the east coast of Taiwan,with a 26-32 degree dip of the Moho.
Highlights
The island of Taiwan is generally considered the result of the collision of the Luzon Arc and the Eurasia continental margin (e.g., Chai, 1972; Suppe, 1981) or a paleo-Ryukyu ArcInstitute for Geophysics, University of Texas, 4412 Spicewood Springs Rd., Bldg. 600 Austin, TX 78759-8500, USATAO, Vol 9, No 3, September 1998(Hsu and Sibuet, 1995) in Late Miocene or Pliocene times
In particular we present an interpretation of ocean bottom seismometer (OBS) data collected along TAICRUST Line 14
On the basis of intersecting line 1 velocity models, which indicate the plate boundary at -18-22 km beneath the Nanao Basin (Wang et al, 1996; Mcintosh et al, 1997), and hypocentral depths of low angle thrust earthquake events at 15-25 km in this area (Kao, 1998), it is likely that the boundary between the subducting Philippine Sea Plate (PSP) and the Ryukyu Arc is near the C2-C3 interface
Summary
The island of Taiwan is generally considered the result of the collision of the Luzon Arc and the Eurasia continental margin (e.g., Chai, 1972; Suppe, 1981) or a paleo-Ryukyu Arc. Have interpreted deformation of the Ryukyu Arc and Okinawa Trough along north to north west oriented dextral slip faults on the basis of bathymetry and magnetic anomaly patterns In this area we use new MCS and ocean bottom seismometer (OBS) data to constrain the crustal structure and plate boundary configuration. This sedimentary body, just west of the Nanao Basin, likely is formed from the accretionary prism material of the Yaeyama Ridge as interpreted on an intersecting MCS profile by Lallemand et al (1997) This deformed unit and the underlying arc basement both have clear westward dip between OBS 20 and 21 posi tions. The absence of coherent reflections in this zone approaching Taiwan and the rough seafloor indicate intense deformation
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