Along-strike variations in volatile cycling control trench curvature associated with back-arc spreading

Trench curvature, as the surface expression of the three-dimensional subduction system, has a close affinity with the subduction dynamics; however, the underlying mechanisms remain enigmatic. Back-arc basins, as natural products of subduction zone evolution, record the development of arcuate trenches. Most modern back-arc basins occur in the western Pacific, where subduction zone trenches commonly exhibit no-linear geometries. Among them, the Japan Sea represents a typical example, characterized by the trench convex toward the subducting plate.

Here, we present major and trace element together with Sr-Nd-Mg isotopic data of back-arc basalts (BABB) drilled along strike in the Japan Sea to explore the potential link between trench curvature and lateral variations in subducted materials. The Nb/Zr ratios of BABB in the central segment increase and subsequently decrease, whereas those in the north show a markedly delayed decrease, which indicates that the central back-arc basin had reached a mature spreading stage. In addition, Nd isotopic values of central BABB show higher than those in the south, indicating a negligible contribution from slab-derived components. This implies that the central back-arc basin is located far away from the trench and experienced nearly complete extension. These observations reveal pronounced along-strike variations in the extent of back-arc spreading, with the northern basin remaining nascent, whereas the central segment has evolved to a mature stage. This is consistent with the observation that the central segment of the trench develops a progressive curvature toward the subducting plate, suggesting that the evolution of back-arc spreading exerts a primary control on trench curvature. In particular, along-strike changes in Mg isotopes reveal the lateral variations in volatile cycling. BABB from the northern region with limited spreading exhibit extremely heavy δ²⁶Mg values (−0.30‰ to +0.34‰), suggesting contributions of water-dominated fluids derived from serpentinite. In contrast, BABB from the central region with mature back-arc spreading show relatively light δ²⁶Mg values (-0.57‰ to 0.06‰), primarily reflecting the involvement of deep subducted carbonates.

The spatial variations in volatile cycling correlate well with the extent of back-arc spreading. Volatiles reduce mantle viscosity and weaken the overlying mantle wedge, thereby regulating mantle rheology. It is noted that the magnitude of this effect varies substantially among different volatile species. Among them, carbon exerts a stronger influence on mantle rheology than water (Fei et al., 2013; Kono et al., 2014). This is consistent with the greater extent of back-arc spreading in the central segment, suggesting that along-strike variations in volatile cycling modulate the mantle rheology, thereby governing the evolution of trench curvature.

This work was financially supported by the National Key R&D Program of China (Grant 2022YFF0801002) and the National Natural Science Foundation of China (Grant 42372065).

References:

Fei et al., 2013, Nature, v. 498, p.213-215.

Kono et al., 2014, Nature Communications, v. 5, p.5091.