Structural, geodetic and seismological evidence for tectonic escape in SW Taiwan

Abstract

Recent structural, geodetic and seismological data in SW Taiwan are analysed and discussed in terms of present-day tectonic escape occurring in response to the active N100° collisional shortening. The escaping area corresponds to the onland extension of the Manila accretionary wedge; this region comprises a rheologically weak, thick muddy cover which is decoupled from the underlying basement by a décollement and which deforms mainly by aseismic creep. It is separated from the northern actual collisional area by a major WNW- to NW-trending structural and kinematic transition zone oblique to the structural grain of the belt, the Chishan Transfer Fault Zone. Geodetic data are further used to define several poorly deforming blocks undergoing nearly uniform displacement velocities and bounded by kinematic discontinuities that fit the major faults, and to determine the present-day across-strike and along-strike motions on these major faults. Although direct onland structural evidence of tectonic escape is poor, reconstruction of Quaternary paleostress patterns demonstrate that this escape probably began during the late Pleistocene, later than in northeastern Taiwan as a result of the southward migration of the collision through time. Offshore structural data help to constrain the geometry and the southern extension of the escaping blocks. Finally, a tentative model of lateral extrusion in SW Taiwan is proposed.