Transient nature of convergent plate boundary dynamics

Abstract

Transient or episodic subduction processes are frequently observed along convergent plate boundaries. Highly time-dependent processes are common in continental collision zones where the entry of buoyant continental crust changes the (driving) force balance. Many other transient processes in oceanic subduction zones are more ambiguous, because there, the driving negative buoyancy of the oceanic plate remains approximately constant throughout time. Therefore, such processes are regularly explained by inhomogeneities (e.g. inherited weaknesses, rheological contrasts) within the involved plates or external ’far-field’ forcing altering the flow or stress field.
This work focuses on three such transient processes, namely, (1) episodic back-arc spreading jumps during ongoing subduction, (2) rapid local rift reversals in the overriding plate during the transition from subduction to collision, and, (3) contrasting styles of crustal relamination and magmatism during large scale continental collision. This work aims to identify governing processes for each case in purely buoyancy driven numerical models with homogeneous rheological flow-laws.
The results offer a new mechanism for back-arc spreading jumps in self-consistent models of a narrow retreating slab. Spreading jumps thus occur purely due to natural interaction of the subducting and overriding plates with neighbouring plates along self-consistent transform faults. The process thereby demonstrates that episodicity can evolve in homogeneous subduction systems. It is further shown that rapid rift reversals in the overriding plate should occur regularly during collision with an irregular continental margin. The stress evolution during diachronous onset of collision is compared to observations of ophiolite or hyperextended margin obduction which are likely to have resulted from such transient stress sequences. In the final chapter, it is shown how contrasting styles of crustal relamination and magmatism in continental collision zones are controlled by slab-steepening which itself is mainly a function of the subducting continental crust’s buoyancy.
The three very distinct processes presented in this work demonstrate purely buoyancy driven transient behaviour is feasible in all stages from ongoing subduction to continental collision - in absence of inhomogeneities and far-field forcing.