Subduction reversal in the accretion complex drives the exhumation of deep subducted mélange in southern Qiangtang, Tibet: Insights from the Mao'ershan detachment fault

doi:10.13745/j.esf.sf.2021.11.12

Abstract

Abstract: The subduction and accretion of oceanic lithosphere may be accompanied by a complex slab motion in depth, and HP rocks are certainly a good proxy to reflect this deep process. Recent studies show that subduction reversal can occur, in particular, in a divergent double subduction zone when a slab pull of one slab exceeds that of a shorter slab, the shorter slab then experiences a net upward pull. This study prompted us to pay attention to this ‘abnormal’ plate movement during oceanic subduction and accretion in objective and comprehensive analysis of collisional orogen belt. Most of the current mechanisms of the exhumation of HP rocks emphasize a single rapid exhumation process, except the ‘corner flow’ model. It can be expected that the exhumation process should be different in the case of subduction reversal. Exhumation of a single HP rock is still a rapid process; but exhumation of an entire HP rock belt must occur during the entire subduction reversal period, so the exhumation process lasts longer. Research on the exhumation related upper crustal structural deformation has the potential to unveil the above process.|||This subduction reversal hypothesis is first proposed for Triassic HP rocks exposed in the southern Qiangtang mélange belt in central Tibet. Therefore, we chose to study the Mao'ershan accretionary complex located in the northernmost part of the Qiangtang mélange belt. We analyzed its crustal structural characteristics, geometric structure, movement style and active period related to subduction reversal, based on geological mapping and structural and chronological studies. The field geological mapping results show that the Mao'ershan complex has similar characteristics as the metamorphic core complex. Subduction-accretion complex forms its core that is surrounded by Late Paleozoic strata from the top, with a detachment fault system separating the two. Beneath the brittle detachment fault, a shear zone develops southward from the top of the subduction-accretion complex. Three-dimensional strain and kinematic vorticity results indicate that the strain type in the Mao'ershan shear zone is elongate strain, dominated by simple shear strain. Mineral deformation analysis and fractal dimension measurements show that the shear temperature is associated with low greenschist and lower amphibolite facies. Based on the new ⁴⁰Ar-³⁹Ar geochronology data, we conclude that the Mao'ershan shear zone was active at ~260 Ma. Based on the above study and combined with the geological features of the central Qiangtang mélange belt, we believe that the Mao'ershan complex was exhumed during the early stage of the subduction reversal. We thus infer that the movement rate of subduction reversal is about 3.5 mm/a, which is similar to the exhumation rate of HP rocks in central Qiangtang mélange belt. Our research may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

Key words: southern Qiangtang accretionary complex, exhumation mechanism of HP rocks, detachment

CLC Number:

P542.2
P548

LI Dian, WANG Genhou, LIU Zhengyong, LI Pengsheng, FENG Yipeng, TANG Yu, LI Chao, LI Yang. Subduction reversal in the accretion complex drives the exhumation of deep subducted mélange in southern Qiangtang, Tibet: Insights from the Mao'ershan detachment fault[J]. Earth Science Frontiers, 2021, 28(6): 205-226.

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