MOBILIZING THE CORE OF CENTRAL RANGE IN TAIWAN OROGENY - GEODYNAMICS

A 2-D numerical model for arc-continent collision has been built in which the constitutive updates used in the model allow us to simulate elastic deformation, viscous (ductile) flow using creep laws, Mohr Coulomb plastic flow, the development of brittle fault zones using frictional weakening with a Mohr-Coulomb yield, and the development of mid-crustal detachment horizon using a semi-brittle yield formulation for the continental crust. Diffusional erosion of topography is built in.

The model geometry and plate motion parameters are determined by available data. Starting with an east-dipping transitional lithospheric subduction system, the Luzon arc approaches the Eurasian continental shelf as time progresses, at a velocity of 5 cm/yr. When the arc first impinges on the continental crust a small sedimentary wedge and a set of normal faults, rooted in a ductile shear zone in the midcrust are formed. The normal fault system is used later as a ramp for the main thrust (expressed as a zone of high strain rate in the density field) to develop a larger wedge. A backthrust develops along the arc basement intersecting at depth with the main thrust in a single point similar to the S point of Willett et al. (1993). Later however, nearly vertical faults form in the center of the belt that persist. Normal fault are reactivated as thrust faults in front of the accretionary wedge. These faults mark the transition between accretion in the frontal wedge and exhumation in front of the deformable backstop. With active erosion, this system allows for underplating and exhumation of the continental crust above the ductile shear that is established during bending and extension at the hinge. In 5 Myr, the ductile shear zone itself is exhumed in the core of the mountain belt. This process forms a characteristic pattern of higher temperature and pressure metamorphism between the frontal wedge and the incoming arc, very similar to what is observed in Taiwan. The continental margin resists subduction and the slab steepens and eventually necks and breaks off at the ocean continent transition of the margin. The mountains grow horizontally and vertically as the exhumation of the ductile shears zone continues, but only when strong erosion is present; without erosion new subduction zone developes.