CRUSTAL SCALE LATERAL DISPLACEMENT IN THE ARCHEAN: IMPLICATION FROM ARCHEAN ALPINE-STYLE NAPPES

We report the structural geometry and stacking sequence of an Archean Alpine-style sub-horizontal fold/thrust nappe pile in the Precambrian basement of the Zanhuang massif, northern China. We separate individual allochthonous forearc affinity fold nappes (Black rock temple and Buddha nappe) from the para-autochthonous to autochthonous continental margin, based on the diverse lithological components and the structural-stratigraphic relationships. Circa 2698 ± 30 Ma picrite-boninite and tholeiite units mark the age of subduction initiation. Detrital zircon from the major decollement constrains nappes emplaced over the passive margin at 2520 Ma. It shows the life cycle of the subduction zone and ocean spanned circa 178 million years; conservative plate velocities of 2 cm/yr yield a lateral transport distance of subducted oceanic crust of 3560 kilometers.

The first-order geometric restoration and field-based anatomy enables us to subdivide this fold/thrust nappe pile into the top-most nappe so far known (Black Rock Temple nappe) and the lowest (Buddha nappe), as well as the ductile shear zones/decollement shear zones below each of them. A subhorizontal mega-scale sheath fold (kilometer in size) formed in the ductile shear zone between the top-most and lowest nappes is illustrated by a three-dimensional fence profile. Given that large shear strain (γ ≥10) is required by the simple shear mechanism for sheath fold generation, the development of the mega-scale sheath fold corresponds to no less than 10 km of subhorizontal displacement in this Archean nappes pile, not taking into account the larger displacements along the thin nappe-bounding decollements.

This work explicitly unravels the nappes displacement accomplished by the combination of rigid body translation along thrusts and internal strain by ductile deformation. The rigid body translation is illustrated by the dominance of nappe gliding on the decollement surface for the crustal-scale tectonic transportation. The internal strain related displacement is illustrated by the high-strain recumbent giant sheath-fold. With the lateral displacement and internal strain of the Archean Zanhuang nappes, similar to those of Phanerozoic nappes, it provides positive evidence for horizontal plate tectonics in the Archean.