Transition in the Mode of Syn-Collisional Continental-Recycling with Secular Cooling of the Earth and its Implications for Crustal Evolution

Continental crustal recycling (CCR) critically controls the physico-chemical evolution of the crust-mantle system of the earth [1]. The different modes by which such recycling may occur on modern earth are recognized and are linked to plate tectonics [2]. However, the tectonic style itself is likely to have been different in an earlier hotter earth [3]. Thus, the mode and extent of CCR is also likely to have been different and extrapolation of modern recycling rates back in time could be erroneous. A systematic exploration of the nature and magnitude of CCR as a function of tectonic styles with changing thermal structure has not yet been carried out. We carried out 2D high resolution thermomechanical modeling to study the syn-collisional recycling styles at various mantle potential temperatures (Tp). With increasing mantle Tp, syn-collisional recycling mechanism changes systematically from slab failure-dominated to delamination-dominated. Viscous strength of the continental lithosphere decreases as mantle Tp increases and thereby, the tendency to delaminate increases as well. Calculation of the recycled crustal volume yielded a flux of 0.3-0.8 km3/a by slab failure and a much higher flux of 1.5-3.6 km3/a by delamination under hotter mantle conditions. Delamination removes the lower continental crust preferentially whereas slab failure recycles both upper and lower continental crust in sub-equal proportions. Linking the mantle Tp with earth's age [e.g. 4], our results predict an extensive delamination led recycling till 1.5 Ga. Subsequently, slab failure would have started to become the more common recycling mechanism at collision zones and consequently, the recycling-rates started to decline. The relative dominance of recycling by delamination (mainly lower crust) vs. slab failure (sub-equal amounts of upper- and lower crust) has major implications for the compositional evolution of the continental crust as a whole. [1] Bowring & Housh 1995, Science 269, 1535-1540. [2] Scholl & von Huene 2009, GSL Sp. Pub. 318, 105-125. [3] O'Neill et al. 2007, EPSL 262, 552-562. [4] Herzberg et al. 2010, EPSL 292, 79-88.