REACTIVATION OF CONTINENTAL MARGIN FRACTURE ZONES IN THE MODERN TAIWAN AND ANCIENT TACONIC COLLISIONAL OROGENIC BELTS

A variety of irregularities may be present in the subducting plate along convergent plate margins, including continental margin fracture zones. Orogenic processes related to the reactivation of partially subducted continental margin fracture zones at mid-crustal levels are analyzed for the modern Taiwan and ancient Taconic collisional orogenic belts using a combination of structural and seismic data.

In Taiwan, magnetic anomaly data define the offset continental crust in the subducting Eurasian plate. The offset coincides with the NNW-trending Sanyi-Puli seismic zone, which separates the high topography of the Hsuehshan Range from the Puli Basin. In the Taconics, a remnant continental margin fracture zone is identified in the Green Mountain massif, its oceanward projection obscured by subsequent tectonic events. The fracture zone is inferred to be part of a system of NW-trending, left-stepping transform faults between the New York promontory and Quebec reentrant.

Deformation in the hanging wall of the reactivated fracture zone in the Taconic allochthon to the west of the Green Mountain massif is described by a change in trend of regional-scale fold axes, strike and dip of cleavage, and rake of the mineral lineation on cleavage. In addition, the strata have undergone relatively low amounts of strain and show evidence of triclinic strain symmetry and flattening strain, in contrast to strata to the north, which are characterized by relatively high amounts of strain, monoclinic strain symmetry, and plane strain. The strain and kinematic data are consistent with the results of theoretical modeling and suggest that reactivation of the fracture zone resulted in the development of a moderately oceanward-dipping oblique ramp in the cover rocks of the Taconic sequence. The Sanyi-Puli seismic zone is not associated with an along-strike change in orientation of stratigraphic units at the surface. Focal mechanism solutions at depth, however, indicate left-lateral displacements. This suggests that reactivation of the fracture zone in Taiwan may be relatively recent or that the zone of deformation has not propagated to the surface.