**4. Subduction zone initiation (SZI)**

Two concepts are commonly proposed for subduction zone initiation (SZI): (i) spontaneous or vertically forced and (ii) induced or horizontally forced SZI [74–76]. Vertically forced SZI is caused by the contrast between the underlying convicting mantle and the cooling lithosphere above. The viability of this scenario is controversial [77], especially for SZI at passive margins [78]. The body force resulting from density differences is most likely insufficient to break up and initiate a subduction zone if the rift basin lithosphere is older than 20 My after the continental breakup [79]. This assumption is not consistent with the ages of descending, hyperextended continental basins, which are generally older than 20 My (e.g., ~34 My for Oligocene subduction of the New Caledonia Basin beneath the northern Norfolk Ridge, SE Pacific; [20] and ~ 60–65 My for initiation of subduction of Piemonte-Liguria Ocean beneath the Adriatic continental margin, [78]. Moreover, data from recent and ancient subduction zones [76, 80] suggest that vertically forced SZI was probably not the dominant scenario during the last 100 million years. Therefore, the horizontally forced SZI model is preferred to overcome the increasing strength of the cooling lithosphere [76, 78, 81]. The far-field external horizontal forces can be caused by the mid-ocean ridge, mantle plume, neighboring sinking slab, and large-scale mantle convection [82]. However, [83] argues that vertical forces can accelerate, propagate, and facilitate the development of self-sustaining subduction zones that are initially dominated by horizontal forces.

It is most likely that the initiation of subduction zones by horizontal compressive forces requires a process that mechanically weakens or softens the lithosphere to aid in the localization stresses that eventually lead to the breakup [20]. Several mechanisms have been proposed for softening the lithosphere during SZI including mineral reaction and transformation [84], fluid-induced [20, 85, 86], microstructural evolution and anisotropy [87], mineral grain damage and plunging [88, 89], and thermal softening [78, 90, 91]. Petrological thermomechanical models show that a temperature increase of only ca. 50°C is sufficient for successful SZI [78, 92]. Therefore, thermal softening is considered a potential mechanism to form a shear zone transecting the lithosphere [90, 93–97] and initiate subduction at a hyperextended continental [78]. Serpentinization within the lithosphere may also serve as a stress conductor during subduction initiation because it is weaker than peridotite, continental crust, and oceanic crust [98–100]. In addition, greater serpentinization would reduce the coherence of the rift basin lithosphere [7], which generally exhibits shallow slab detachment (SD) early after the SZI [78]. In contrast, the low-serpentinized lithosphere is coherent and requires more compressional forces for SZI [79, 101]. The strong lithosphere may be able to maintain a continuous subducting slab down to 660 km depth for more than 20 Myr after basin closure [78].
