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 (γ), compression ratios (c%) and octahedral strain intensity (ε(s)) are similar across the Jiaochang structure (i.e., γ ≈ 1.8, c ≈ 27%, ε(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 (D(1)–D(2)) during the Indosinian tectonic epoch, with insignificant post-D(2) deformation.