ON THE DIP ANGLE OF SUBDUCTING PLATES

An approximate analytic model to study the thermal structure of a subducting plate with finite length has been developed. The model is able to take into consideration the effects of a moving front edge and adiabatic compression of slab material. Based on this model, torque balance upon a descending slab is examined, and the resultant implications to the evolution of slab dip angles are investigated. Thermal structure within a descending slab of finite length is found to be similar to that predicted by a steady state assumption except near the slab tip. The effect of the leading edge is felt only within a distance equivalent to the slab thickness. Away from this region, slab temperature is largely dependent on the subduction velocity, the distance from the surface and the initial temperature profile of the plate. On the basis of this analytic thermal model, gravitational torques acting on subducting slabs can be evaluated for a complete range of dip angles. A comparison with field observations suggests that short slabs (i.e. slabs with penetration depth ≤ 400 km) are likely under torque equilibrium at present. For slabs that have penetrated the olivine-spinel phase boundary at about 400 km depth, the increase of gravitational torque due to the upward deformation of the phase boundary forces the slabs to move away from equilibrium. As a result, long slabs are speculated to be transient at present, rotating towards steeper dip angles. This transient behavior is likely to cause the mantle wedge to be pulled downward, and may explain the observed deeper bathymetry within many back-arc basins relative to normal sea floors of the same age. © 1990.