Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective

Basalts and picrites from a plume-type ophiolite in the South Qilian Accretionary Belt, Qilian Orogen: Accretion of a Cambrian Oceanic Plateau?

“…When a subducting ocean plate contains a body that is too buoyant to subduct the phenomenon of "trench jam" can occur (e.g., Abbott et al, 1997;Niu et al, 2003Niu et al, , 2015, with the consequence that a new subduction zone and volcanic arc are generated. Oceanic plateaus are the best candidates for such a buoyant and unsubductable mass, given the relatively low densities of the high percentage melts present in such crust (Herzberg, 1999;Niu and Batiza, 1991;Niu et al, 2003; O'Hara, 1973).…”

“…Oceanic plateaus are the best candidates for such a buoyant and unsubductable mass, given the relatively low densities of the high percentage melts present in such crust (Herzberg, 1999;Niu and Batiza, 1991;Niu et al, 2003; O'Hara, 1973).…”

Testing the geologically testable hypothesis on subduction initiation

“…Therefore, there would be no plate tectonics if there were no subduction zones [6]. Yet how a subduction zone begins remains speculative [4].…”

“…Studies on subduction initiation have been many and continue to this day by using modeling (both kinematic and dynamic), geological and petrological approaches [e.g., 4,6-11], culminating with three consecutive IODP drilling expeditions (IODP 350, 351 and 352 in 2014) in the western Pacific to test the ideas of spontaneous and induced subduction initiation [10]. Niu and co-authors [4,6] advocate with quantitative illustrations that subduction initiation is a consequence of compositional buoyancy contrast within the lithosphere. All these studies attempt to explain how a subduction zone may initiate (not about when and how plate tectonics began in Earth's history).…”

“…Thermal buoyancy contrast in the lithosphere is attractive [7], but no large scale linear thermal buoyancy contract exists on the Earth except the Romanche transform in the equatorial Atlantic across which the ridge encounters old lithosphere of ~ 75 Ma, where large thermal buoyancy contrast must exist, but there is no sign of subduction initiation [6]. Subduction initiation along prior zones of weakness such as earlier transforms, fracture zones and failed seafloor spreading centers has been a popular idea [9][10][11], but it is physically difficult to understand why and how one side of these features would prefer to sink while the other side would choose to rise under any deviatoric stresses [6]. The Macquarie Ridge, marking the boundary between the Australian and Pacific plates in the Southern Ocean, has been considered by some as the most convincing case of incipient initiation of a subduction zone, but again there is no sign of subduction initiation [6].…”