Paleozoic Trench Migration in the Chinese Eastern Tianshan (Central Asia): Constrained by Crustal Thickness Reconstruction

Abstract

Trench migration is a first‐order control on convergent plate boundary evolution, but reconstructing its deep‐time history remains challenging due to orogenic overprinting. Here we apply recently developed crustal‐thickness proxies (zircon Eu/Eu* and (LREE/HREE) _N ) to the Chinese Eastern Tianshan Orogen (Central Asia), where the North Tianshan intra‐oceanic arc system lay north of the Central Tianshan Block, separated by the now‐closed North Tianshan Ocean. These proxies reflect pressure‐dependent mineral fractionation and thus track relative changes in crustal thickness. By tracking overall increases and decreases in these proxies, we reconstruct Paleozoic crustal thickness variations, which are a key diagnostic of trench migration because crustal thickening accompanies trench advance and thinning reflects retreat. Our results reveal that subduction of the North Tianshan oceanic slab beneath the North Tianshan island‐arc system involved trench retreat at ∼335–315 Ma, followed by trench advance at ∼315–300 Ma. In contrast, subduction beneath the Central Tianshan Block records an earlier advance at ∼450–430 Ma, a retreating phase at ∼410–360 Ma, and a renewed advance at ∼360–320 Ma. These trench‐migration patterns, inferred from crustal thickness variations, are consistent with independent geological records of deformation, sedimentation, and magmatism, validating the use of crustal‐thickness reconstructions for tracing deep‐time trench migration in both mature intra‐oceanic and continental arc systems. A comparison with Hf‐isotopic trends reveals that Hf signatures are not solely controlled by crustal thickness but are also modulated by crustal composition and other geodynamic processes, suggesting that Hf isotopes should be used with caution when tracking trench migration.