Supercomputer Simulation of Continental Collisions in Precambrian: The Effect of Lithosphere Thickness

摘要

Many aspects of Precambrian tectonics are still unclear due to the undetermined influence of several key physical parameters (the temperature of the mantle, the thickness of the lithosphere, etc.) on the geo-dynamic processes. The values of these parameters in the Precambrian are known to have been significantly different from current conditions. This work presents the results of two-dimensional (2D) numerical petrological-thermomechanical experiments that simulate the process of plate convergence at a velocity of 5 cm/year depending on the thickness of the continental lithosphere. The model continental-lithosphere thickness ranged from 100 to 200 km, the modern mantle temperature exceeded 150℃, while radiogenic heat generation in the continental crust was 1.5 times higher then that at present. The numerical simulations showed that if the lithosphere is 100-160 km thick, the subduction process (closure of an ocean) is terminated by detachment of the oceanic plate from the continental plate (slab break-off) followed by the formation of a large igneous province (an oceanic plateau) between the continents instead of orogeny. The thinner the lithosphere is, the earlier and closer to the surface the slab break-off occurs. Thus, for a model with a continental lithosphere thickness of 150 km, the slab was detached in 10.3 Ma at a depth of 150 km, whereas for a lithosphere of 100 km thick it occurred in just 5.1 Ma almost at the surface. In the latter case, the magma generation area becomes much larger due to the formation of igneous provinces on both sides of the oceanic slab instead of one side, as is proposed in other models. Collision of continents with a very thick lithosphere (200 km or more) is not accompanied by slab break-off and significant volcanic activity. Thus, the results of our modeling show that the SLAB PULL mechanism in a subduction zone contributes to the processes at a plate convergent boundary.