What conditions promote atypical subduction: Insights from the Mussau Trench, the Hjort Trench, and the Gagua Ridge

Abstract

Subduction zones act as interfaces for exchanging materials between the Earth’s crust and mantle. The western Pacific plate region is evolving within a convergent tectonic environment. Old oceanic plates generally subduct beneath young oceanic plates, as exemplified by the Izu-Bonin-Mariana and Tonga-Kermadec subduction zones. However, in some subduction zones, such as the Mussau Trench, the Hjort Trench, and the Gagua Ridge, young and more buoyant oceanic plates have been recognized to subduct underneath older and denser plates. What conditions promote atypical subduction, however, remain elusive. In this study we take the Mussau Trench, the Hjort Trench, and the Gagua Ridge as examples to explore the possible underlying factors that control the formation of atypical subduction. By anatomizing the tectonic features of both the Mussau Trench and the Hjort Trench, we find that atypical subduction may be feasible mainly when the plate boundary is characterized by strike-slip-dominated transpression; that is, the strike-slip component overwhelms the compression component, which may argue against the atypical subduction occurring at the Gagua Ridge, at least at present. Moreover, in light of the evolution of both the Mussau Trench and the Hjort Trench, it is further suggested that subduction polarity reversal and a strike-slip border are the keys to atypical subduction.