Enigmatic Pre-collisional Extensional Tectonics in Microcontinents

Subduction zones are active plate boundaries not only for consuming oceanic plates but also accretion of microcontinents (terranes) prior to collisional events. While it has been shown and widely accepted that these terranes might show extensional tectonics following a collisional event due to the post-orogenic collapse, recent geological evidence has discovered that in certain cases microcontinents may have undergone a significant phase of extension prior to plate accretion. Specifically, according to petrological studies, the mid-crustal rocks of a microcontinent in Eastern Anatolia exhumed to the surface between 83 and 71 Ma prior to the accretion at 50 Ma (Topuz et al., 2017). Additionally, a similar pre-collisional extensional event happened in the western Alps, Sesia zone, 20 Ma prior to the collision. To understand this enigmatic pre-collisional extension in microcontinents, we set up a series of numerical geodynamic models and tested various convergence rates and geometries of the microcontinents. According to our model results, a style of slab-pull is playing an important role in the microcontinent extension. The findings quantify a new type of plate deformation that focuses extension on the subducting pro-plate, whereas most subduction systems are characterized by retro-plate extension. The numerical analyses also show that increasing microcontinent widths and plate convergence rates are inversely proportional with calculated exhumation rates. The reason why oceanic lithospheres do not reveal any extensional deformation while microcontinents do, can be explained by their relative strengths. Namely, continental lithosphere might be considerably weaker than oceanic lithosphere and this makes them preferable locations for extensional deformation.