Possible effects of collisions on plate motions

Abstract

A large-scale collision at a plate boundary is expected to play an important role not only in the deformation at the boundary but also in the motion of the plate carrying the buoyant material to be accreted. Possible changes in rates and directions of such motions may be calculated provided that certain assumptions are made about the nature of the driving forces. In this model we shall assume basically that: 1. (1) an oceanic plate is driven by slab pull and ridge push, being resisted by basal asthenospheric drag and slab resistance; and 2. (2) because of detachment, slab pull is lost upon collision. If, however, the calculated motion following collision has a convergent component at the boundary, a new subduction zone, with an increasing slab pull force, forms seaward of the accreting buoyant material. Calculations were first made on an idealized planar octagonal plate. Results indicated that, so long as the scale of collision is limited, changes in motion take place but the motion returns to the initial state. However, for large-scale collisions, the plate motion suffers a large directional change, which also changes the nature of some boundaries, and hence the motion approaches a new steady-state motion irreversibly. Calculations were then conducted on an idealized Philippine Sea plate on a spherical earth. For a sizable collision along the Taiwan-Philippine-Mindanao boundary, this boundary and perhaps the Bonin-Mariana boundary, may become a transform fault provided that slab pull-ridge push is the driving force. These results provide some support to Uyeda-McCabe model (1983) of episodic spreading of the Philippine Sea basins, as well as the predominance of strike-slip motion along collision boundaries. Finally, in agreement with prior studies, subduction of a ridge system, assumed to have been located along the southwestern rim of the Paleo-Pacific plate, was shown to be consistent with the change in direction of motion of that plate at 43 m.y.B.P. Calculations conducted in this study show that predicted changes are model dependent so that comparisons with observed or inferred motions may be useful as a means of estimating the relative importance of the various potential driving forces.