Giant sheath-folded nappe stack demonstrates extreme
subhorizontal shear strain in an Archean orogen
Yating Zhong1, Timothy M. Kusky1,2* and Lu Wang1
1 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
2 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.
