Differentiating induced versus spontaneous subduction initiation using thermomechanical models and metamorphic soles

Abstract

Despite the critical role of subduction in plate tectonics, the dynamics of its initiation remains unclear. High-temperature low-pressure metamorphic soles are vestiges of subduction initiation, providing records of the pressure and temperature conditions along the subducting slab surface during subduction initiation that can possibly differentiate the two end-member subduction initiation modes: spontaneous and induced. Here, using numerical models, we show that the slab surface temperature reaches 800–900 °C at ~1 GPa over a wide range of parameter values for spontaneous subduction initiation whereas for induced subduction initiation, such conditions can be reached only if the age of the overriding plate is <5 Ma. These modeling results indicate that spontaneous subduction initiation would be more favorable for creating high-temperature conditions. However, the synthesis of our modeling results and geological observations indicate that the majority of the metamorphic soles likely formed during induced subduction initiation that involved a young overriding plate.

Boron isotopes in boninites document rapid changes in slab inputs during subduction initiation

“…We have calculated trace element abundances for mineral-melt assemblages of representative amphibolite samples from the slab-mantle interface beneath the Oman ophiolite, one with high and one with low Ba/Nb (samples WT28A-5 and WT32 34 ), under hornblende-bearing granulite facies of 900 °C and 0.9 GPa. This temperature is slightly higher than the reported peak metamorphic temperature of ~825 °C beneath the Oman ophiolite 46 but is consistent with recent modeling by Zhou and Wada 25 . The chosen pressure is comparable to the highest equilibrium pressure estimates for LSB genesis (0.4–0.9 GPa 12 ).…”

“…In mature subduction zones, the δ 11 B of the altered oceanic crust near the slab-mantle interface decreases due to early losses of high δ 11 B boron to the forearc mantle at shallow depth, as indicated by δ 11 B of −6 ± 4‰ of mafic blueschist clasts metamorphosed at ~19 km depths and 200–350 °C, recovered from serpentinite muds in the Mariana forearc 33 . However, during subduction initiation, the downgoing tip of the slab reaches much higher temperatures, upwards of ~900 °C at 1 GPa based on recent models 25 . B, Rb, K, and Ba concentrations are high in amphibolites in the metamorphic sole of the Oman ophiolite, suggesting exchanges with FME-rich fluids during prograde slab metamorphism 34 .…”

“…Subduction initiation as recorded in the IBM system 24 reflects uniquely high temperature, low pressure conditions 25 , as initial slab foundering led to extension, asthenospheric upwelling and melting, resulting first in FAB magmatism and then boninite magmatism, all occurring in close proximity to the slab edge, such that shallow, hot mantle depleted by FAB melting was re-melted due to fluxing by slab-derived inputs 4 , 12 , 13 , 16 . Hot mantle conditions near the sinking edge of the downgoing plate are likely responsible for the generation of melts in the slab crust, suggested to occur under hornblende-bearing granulite facies conditions (900–950 °C 16 , 26 ).…”

Subduction Initiation at the Solomon Back‐Arc Basin: Contributions From Both Island Arc Rheological Strength and Oceanic Plateau Collision

“…Our results find that if the old slab has been broken‐off, it can induce mantle flow to weaken the overlying plate (Figure S6 in Supporting Information ). However, the weakening is not sufficient to cause lithospheric break‐up and onset of subduction in the youngest Solomon Sea Basin (about 40‐25 Ma) with a lower lithospheric density (Maunder et al., 2020; Zhang & Leng, 2021; Zhou & Wada, 2021). And, it is worth noting that the SI in our reference model is preceded to slab detachment caused by the newly subducting slab (Figure 2) rather than the old slab break‐off.…”

Horizontal Force Required for Subduction Initiation at Passive Margins With Constraints From Slab Detachment

“…Published numerical models of SZI can be classified in two categories: 1) Models that investigate SZI at already existing, weak plate boundaries, which are prescribed as weak zones in the initial model configuration. These studies focus on the onset of sinking of a lithospheric plate and sometimes on the subduction of one plate below another plate (e.g., Malatesta et al, 2013;Zhong and Li, 2020;Zhou et al, 2020;Zhou and Wada, 2021). These studies do not investigate the formation of a new convergent plate boundary associated with SZI.…”