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Giant sheath-folded nappe stack demonstrates extreme subhorizontal shear strain in an Archean orogen
Date: 2022-03-24      SourceLink:      ClickTimes:
Giant sheath-folded nappe stack demonstrates extreme  
subhorizontal shear strain in an Archean orogen
Yating Zhong1, Timothy M. Kusky1,2* and Lu Wang1
State Key Laboratory of Geological Processes and Mineral Resources, and Center for Global Tectonics, School of Earth Sciences,  
China University of Geosciences, Wuhan 430074, China
Badong National Observation and Research Station of Geohazards, China University of Geosciences, Wuhan 430074, China


ABSTRACT

Giant sheath-folded nappes are associated with suture zones and emplacement of far  traveled allochthons in Phanerozoic orogens, demonstrating a rare but significant geologic phenomenon indicative of modern-style plate tectonics. We document the world’s oldest known subhorizontal mega-scale sheath fold from Archean Alpine-style nappes of the Central orogenic belt, North China craton. The Zanhuang nappes are recumbent Alpine-style forearc affinity metabasaltic and metasedimentary nappes emplaced over a passive continental margin in the Archean, marking an ancient suture zone. Field evidence shows multiscale sheath folds from decimeters to tens of meters in size, and our three-dimensional fence profile, fold hinges,  kinematiclineations, and lithological traces define an ∼1-km-long (parallel to the x-axis)   sheath fold in the core of the nappem stack. Structural analysis statistically demonstrates the macro-scale recumbent sheath-folded nappe preserves a complete 180° hinge-line curvature.  The giant sheath fold plunges northwest, reflecting its formation during non-coaxial, top o-the-southeast shearing with extremely high shear strain (γ ≥10), equated to >10 km of ductile slip on the bounding surfaces. Slip vectors derived from S-C fabrics on overturned  
limbs are consistent with rotation into the southeast-directed transport direction, parallel  to the similarly rotated fold hinges. Comparison of the giant sheath-folded nappes from the Archean Zanhuang example with mega-scale sheath folds in Phanerozoic and Proterozoic orogens shows that Neoarchean lithosphere was stiff enough to allow tectonics to operate in a manner analogous to modern-style plate tectonics.


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