3 years ago

Subduction reversal in a divergent double subduction zone drives the exhumation of southern Qiangtang blueschist‐bearing mélange, central Tibet

D. Li, G.H. Wang, P.D. Bons, Z.B. Zhao, J.X. Du, S.L. Wang, G.L. Yuan, X. Liang, L. Zhang, C. Li, D.R. Fang, Y. Tang, Y. Hu, Y. Fu

(Ultra) high‐pressure (HP) rocks can be exhumed rapidly by subduction inversion or divergent plate motion. Recent studies show that subduction inversion can in particular occur in a divergent double subduction zone when the slab pull of one slab exceeds that of the other, shorter one, which then experiences a net upward pull. This recent hypothesis, first proposed for Triassic HP‐rocks exposed in the Central Qiangtang mélange belt in central Tibet, can explain the exhumation of (ultra) high‐pressure rocks through upward slab movement. However, this model lacks the support of kinematic evidence. In this study, based on the recognition of multiple deformational phases, we analyze the kinematics of the HP‐bearing mélange in central Qiangtang. Based on new 40Ar‐39Ar geochronology data and those collected from the literature, we present a temporal framework for the new observations. We recognize a switch in sense of shear between the prograde (D1) and exhumation (D2‐3) paths. The change of shear sense reflects the reversal from downward to upward movement of the oceanic slab below. Early D2 represents the early exhumation stage that caused retrograde metamorphism from eclogite to blueschist facies. No magmatism occurred during this period. Continued exhumation from blueschist facies to greenschist facies resulted in D2‐D3 structures. Voluminous igneous activity occurred during this stage. We suggest that subduction inversion in a divergent double subduction zone can best explain the kinematic evolution and temporal framework above. This exhumation model may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

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