SUBDUCTION VERSUS ACCRETION OF INTRA-OCEANIC VOLCANIC ARCS: INSIGHT FROM THERMO-MECHANICAL ANALOGUE EXPERIMENTS

Abstract

We perform thermo-mechanical laboratory experiments designed to explore the behaviour of the volcanic arc during intra-oceanic arc–continent collision following oceanic subduction and subsequent back-arc opening. The overriding oceanic lithosphere is made of two layers representing the oceanic crust and the lithospheric mantle. This lithosphere carries a volcanic arc and is thinned and weakened beneath both the arc and the back-arc basin. The subducting plate contains three parts: one-layer oceanic and two-layer (crust and mantle) continental lithosphere with a continental margin between them. When the continental margin reaches the trench and starts subducting, the overriding plate undergoes growing horizontal compression and finally fails in the vicinity of the back-arc spreading centre, which is the weakest part of this plate. The failure can result in subduction of the whole arc plate comprised between the trench and the back-arc spreading centre. During subduction of the arc plate, the mantle part of this plate subducts completely, while the behaviour of the arc crust depends on its thickness and strength, which is a function of composition and temperature. We tested four cases with different arc crust thicknesses and composition (rheology), with total lithosphere thickness in the arc being constant. Three types of tectonic evolution have been obtained: complete arc subduction, complete arc accretion, and partial arc subduction/accretion. The result is largely controlled by the crustal thickness of the arc. A thin arc (thickness equivalent to ∼16 km in nature) made of the same strong material as the oceanic crust subducts completely without leaving any trace at the surface. On the contrary, a thick arc (equivalent to ∼26 km in nature) made of the same material is scraped off and accreted to the overriding plate. The lower crust of such an arc is hotter, therefore its strength at ‘Moho’ depth and coupling between crust and mantle are small. In addition, the thick arc has a high isostatic relief and hence a greater mechanical resistance to subduction. Therefore, the arc is scraped off. If the arc is made of a weaker ‘continental-like’ material or contains a weak layer/low friction interface, it is completely or partially scraped off even if it is small. When there is no back-arc opening before collision (no thin and weak lithosphere in the rear of the arc), the overriding plate fails in the arc area, which may result in a complete fore-arc block subduction, with the volcanic arc remaining at the surface. The obtained models are compared with mountain belts with nearly no trace of arc activity (Oman), with accreted arc (Kohistan), and with small remnants of subducted arc (southern Tibet).