Dynamics of multiple microcontinent accretion during oceanic subduction

Microcontinent accretion during oceanic subduction is one of the main contributors to continental crustal growth. Many of the continental mountain belts we find today were built from accretionary orogenesis, for example, the Cordillera of the west coast of the Americas, the European Alps, and the Australian Lachlan orogen. Continental growth can also be observed in modern accretionary orogens such as the Pacific accretionary belt, with the collision of the Philippine microplate, and the Taiwan-Luzon-Minduro Belt. In many of these systems, multiple bathymetric highs, such as microcontinental terranes, island arcs, or oceanic plateaus, are accreted before full oceanic closure, thus significantly altering the subduction zone before continental collision occurs.The process of accretion implies a complex balance of multiple geodynamic forces that can result in either microcontinent subduction, microcontinent accretion, or subduction stalling (which could lead to the initiation of an altogether new subduction zone). The most important driving forces in this system are the slab-pull force arising from the negative buoyancy of the down-going slab and the far-field force which is the result of large-scale plate-motions external to the subduction zone. These forces are counteracted (among others) by friction along the subduction interface and the buoyancy of the downgoing microcontinent. The resulting net forces control the overall stress-field of the overriding plate as well as the state of stress and potential deformation of any further microcontinents embedded within the oceanic lithosphere that are not yet in the subduction zone. When multiple microcontinents are embedded in the subducting oceanic plate, the friction along the subduction interface and its temporal variations can take a crucial role. The accreting microcontinents have a first order effect on the length and the rheology of the subduction channel, thereby controlling the interface friction. The fate of the microcontinents (e.g. full or partial accretion, or subduction) also affects the overall buoyancy of the slab, altering the balance of forces through the slab-pull.Using 2D thermo-mechanical experiments with the finite-element software SULEC-2D, we explore the roles of the structure and rheology of multiple accreting microcontinents (controlling their integrated strength) as well as the velocity of the subducting plate (controlling the far-field and the slab-pull force) to better understand how accretion of crustal units can modify the subduction zone and affect later continental collision. Our setup is comprised of a subducting oceanic basin surrounded by two continents. In this setup the oceanic plate is either “empty” or one or two microcontinents are embedded within it.Our first results show that microcontinent accretion is promoted by the presence of a weak rheological detachment layer within the microcontinent. In turn, strong coupling of the microcontinental crust to its host-lithosphere promotes terrane subduction and may ultimately lead to the stalling of subduction. Moreover, the behavior of the microcontinents during accretion and subsequent continental collision has a first order effect on the structural style of the resulting orogen as the rheology of the microcontinents controls the degree of localization of deformation in the subduction channel.