Progressive Indosinian N-S Deformation of the Jiaochang Structure in the Songpan-Ganzi Fold-Belt, Western China

Integrated field data, microstructural and three-dimensional strain analyses are used to document coaxial N-S shortening and southward increase in deformation intensity and metamorphism at the Jiaochang structure. Two episodes of deformation (D-1,D-2) with localized post-D-2 deformation have been identified in the area. The first deformation (D-1) episode is defined by a main axial-plane of parallel folds observable on a micro-to kilometer-scale, while the second episode of deformation (D-2) is defined by micro-scale metamorphic folds, associated with E-W oriented stretching lineation. These processes are the result of Indosinian tectonism (Late Triassic to Early Jurassic) characterized by nearly coaxial N-S compression and deformation. This is indicated by E-W trending, sub-parallel to parallel foliation (S-1, e. g. axial-plane of folds, and S-2, i.e. axial-plane of metamorphic folds, crenulation cleavage) and lineation (L-1, e. g. axis of folds, and L-2, i.e. stretching lineation, axis of metamorphic folds and B-axis of echelon lens). Most of the porphyroblasts and minerals (e. g. pyrite, biotite) show two growth phases with localized growth in the third phase (muscovite). The progressive D-1-D-2 structure is widespread in the south of the Jiaochang area, but only D-1 structure crops out at the north. The strain intensity (c), compression ratios (c%) and octahedral strain intensity (epsilon(s)) are similar across the Jiaochang structure (i.e., gamma approximate to 1.8, c approximate to 27%, epsilon(s) = 0.9), showing a broad range of Flinn values (K = 0.77 to 7.57). The long-axis orientations are roughly symmetric between two limbs of the structure. Therefore, we suggest that the architecture of the Jiaochang structure has been controlled by coaxial N-S shortening and deformation (D1-D2) during the Indosinian tectonic epoch, with insignificant post-D-2 deformation.