Dynamics of terrane accretion during seaward continental drifting and oceanic subduction: Numerical modeling and implications for the Jurassic crustal growth of the Lhasa Terrane, Tibet

The oceanic plateau subduction and resulted terrane accretion have played significant roles in the continental crustal growth; however, the modes and dynamics of plateau accretion during subduction remains poorly understood. The oceanic subduction can be driven by the combined effects of slab pull, ridge push, as well as the possible overriding plate push. The latter one has been rarely studied, but can be the case during the multiple Tethyan slabs subduction and terranes accretion for the formation of Tibetan lithosphere in the Mesozoic. Using 2-D petrological-thermo-mechanical models, we systematically investigated the modes and dynamics of terrane accretion during the dominant seaward continental drifting and oceanic plateau subduction. The model results show that the plateau with continental crustal affinity favors accretion to the overriding plate during the subduction process. Three distinct terrane accretion modes are identified: frontal accretion, bottom accretion, and a transitional mode with both the frontal and bottom accretion. In contrast, the plateau with oceanic crustal affinity generally favors subduction and recycling into the deeper mantle, rather than accretion, which is mainly due to the higher density and possible eclogitization of the oceanic crustal materials during subduction. The numerical simulations further reveal that the controlling factor for the subduction style, i.e. flat versus steep subduction, is the age of the oceanic slab and the seaward overriding plate motion. The rheological properties and the convergence rate play minor roles in affecting the subducting slab angle. The model results are further compared with the Jurassic magmatic rock distribution and geochemical characteristics of the Lhasa Terrane in Tibet, which reveals (1) the transition from flat to steep subduction of the southward Bangong-Nujiang Tethyan slab; and (2) the mechanism of Jurassic crustal growth in the central Lhasa subterrane which may be strongly correlated with the bottom accretion of the subducted oceanic plateau.