STRAIN PARTITIONING AND BLOCK EXTRUSION IN THE TAIWAN ARC-CONTINENT COLLISION

The Taiwan orogenic system forms as a result of oblique collision of the Luzon arc with the Asian continental margin and as the Philippine Sea plate converges obliquely with the strike of the orogen. The obliquity between the arc and the margin results in the progressive closure of the South China Sea (SCS), southward propagation of the orogenic system at a rate of approximately 60 km/Ma, and a time-for-space equivalence along strike. In the vicinity of Taiwan, global plate motion models (NUVEL-1) predict that the Philippine Sea plate moves northwest (~305°–310°) at a rate of ~70 km Ma-1 relative to the Eurasian plate (Seno et al., 1993); recent GPS data support a similar orientation (~305°–310°) (Yu et al., 1997). As a result, the plate convergence vector is ~20° clockwise with respect to the orthogonal to the mountain belt, resulting in the potential for strain partitioning within the orogen. In order to better understand how these different collisional components (oblique collision and oblique convergence) affect strain and deformation in the collision zone, we completed a more detailed analysis of the geodetic data and compared the results with available geological and geophysical data. Based on preliminary analyses, both rigid and non-rigid blocks are identified. One of the largest blocks occurs in the southern Central Range and encompasses a previously recognized zone of low seismicity. The block also correlates, at least locally, with regions of high p-wave velocity and high seismic attenuation and includes the only large exposures of mafic to ultramafic rocks in Taiwan. Based on available GPS data the block appears to be relatively rigid but is moving south-southwest with respect to the colliding arc. An additional, apparently less rigid block appears to be moving southwest with respect to the southern Central Range block, producing an area of extensional strains. The area of extension, however, is occupied by some of the highest and most rugged topography in Taiwan, suggesting substantial vertical motions. These preliminary results suggest that vertical and lateral extrusion, rather than strain partitioning, dominates the kinematics of the Central Range.