Structural and thermal evolution of an Archean fold/thrust nappe belt based on quantitative structural profile

     We report studies of an ancient, ~10 km wide fold/thrust nappe belt developed as one zone of a 150 km wide, Archean collisional orogen in the North China Craton. Principles of cross-section construction and balancing were utilized to derive the first-reported, detailed map and crustal-scale profile of the Archean fold/thrust nappe belt. Combined with stratigraphy and geometric restoration, we demonstrate that the fold/thrust nappe pile consists of four major 2.69-2.52 Ga sub-horizontal nappes stacked upon each other, and ductile shear zones below each of them, emplaced onto a 2.56-2.51 Ga passive margin-like shallow marine platformal para-autochthon at 2.51-2.50 Ga. Four individual nappes were derived from different tectonic belts of an ancient paleo-ocean before they were emplaced. They are characterized by distinctive kinematic and thermal evolution.

    Geometric, statistical analysis of structural-fabrics and kinematic indicators illustrate that the fold/thrust nappes of forearc and oceanic floor sequences have been reworked by folding and thrusting with stratigraphic repetition during three kinematic phases, and the final emplacement was accomplished by a combination of rigid body translation along shear zones between nappes and the para-authochthon as well as internal strain by ductile deformation. The para-autochthon only records the predominant D₂ event, related to the fold/thrust nappes emplacement, but lacks the records of oldest kinematic phase.

    Thermal characteristic inferred by metamorphic studies reveal an inverted thermal structure in the fold/thrust nappes, indicated by the observation of a decrease in metamorphic grade from amphibolite facies in the nappes to greenschist facies in the underlying schistes lustrés forming the basal décollement; but it shows a downward temperature-increase in the metamorphic field gradient in the passive margin para-authochthon with relatively stable pressure records.

    This work explicitly unravels the first-order geometry and the tectono-structural evolution of the 2.5 Ga fold/thrust nappe belt by constraints on the scale, structural sequence, and metamorphic grade of the different tectonic units. This provides a quantitative basis for drawing analogies between Archean and present lithospheric deformation and discussing the style of tectonism during Archean orogenesis.